Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/runtime/gc/accounting/heap_bitmap.h b/runtime/gc/accounting/heap_bitmap.h
index a12809e..f4b725c 100644
--- a/runtime/gc/accounting/heap_bitmap.h
+++ b/runtime/gc/accounting/heap_bitmap.h
@@ -103,8 +103,7 @@
   void ReplaceObjectSet(SpaceSetMap* old_set, SpaceSetMap* new_set)
       EXCLUSIVE_LOCKS_REQUIRED(Locks::heap_bitmap_lock_);
 
-  HeapBitmap(Heap* heap) : heap_(heap) {
-  }
+  explicit HeapBitmap(Heap* heap) : heap_(heap) {}
 
  private:
 
diff --git a/runtime/gc/accounting/mod_union_table-inl.h b/runtime/gc/accounting/mod_union_table-inl.h
index 32ac95f..9ea74d4 100644
--- a/runtime/gc/accounting/mod_union_table-inl.h
+++ b/runtime/gc/accounting/mod_union_table-inl.h
@@ -28,8 +28,7 @@
 // A mod-union table to record image references to the Zygote and alloc space.
 class ModUnionTableToZygoteAllocspace : public ModUnionTableReferenceCache {
 public:
-  ModUnionTableToZygoteAllocspace(Heap* heap) : ModUnionTableReferenceCache(heap) {
-  }
+  explicit ModUnionTableToZygoteAllocspace(Heap* heap) : ModUnionTableReferenceCache(heap) {}
 
   bool AddReference(const mirror::Object* /* obj */, const mirror::Object* ref) {
     const std::vector<space::ContinuousSpace*>& spaces = GetHeap()->GetContinuousSpaces();
@@ -48,8 +47,7 @@
 // A mod-union table to record Zygote references to the alloc space.
 class ModUnionTableToAllocspace : public ModUnionTableReferenceCache {
  public:
-  ModUnionTableToAllocspace(Heap* heap) : ModUnionTableReferenceCache(heap) {
-  }
+  explicit ModUnionTableToAllocspace(Heap* heap) : ModUnionTableReferenceCache(heap) {}
 
   bool AddReference(const mirror::Object* /* obj */, const mirror::Object* ref) {
     const std::vector<space::ContinuousSpace*>& spaces = GetHeap()->GetContinuousSpaces();
diff --git a/runtime/gc/accounting/mod_union_table.cc b/runtime/gc/accounting/mod_union_table.cc
index 05b68c4..aa02f82 100644
--- a/runtime/gc/accounting/mod_union_table.cc
+++ b/runtime/gc/accounting/mod_union_table.cc
@@ -117,8 +117,8 @@
 
 class ModUnionScanImageRootVisitor {
  public:
-  ModUnionScanImageRootVisitor(collector::MarkSweep* const mark_sweep) : mark_sweep_(mark_sweep) {
-  }
+  explicit ModUnionScanImageRootVisitor(collector::MarkSweep* const mark_sweep)
+      : mark_sweep_(mark_sweep) {}
 
   void operator ()(const Object* root) const
       EXCLUSIVE_LOCKS_REQUIRED(Locks::heap_bitmap_lock_)
diff --git a/runtime/gc/accounting/mod_union_table.h b/runtime/gc/accounting/mod_union_table.h
index 5435625..d46281c 100644
--- a/runtime/gc/accounting/mod_union_table.h
+++ b/runtime/gc/accounting/mod_union_table.h
@@ -49,11 +49,9 @@
 // cleared between GC phases, reducing the number of dirty cards that need to be scanned.
 class ModUnionTable {
  public:
-  ModUnionTable(Heap* heap) : heap_(heap) {
-  }
+  explicit ModUnionTable(Heap* heap) : heap_(heap) {}
 
-  virtual ~ModUnionTable() {
-  }
+  virtual ~ModUnionTable() {}
 
   // Clear cards which map to a memory range of a space. This doesn't immediately update the
   // mod-union table, as updating the mod-union table may have an associated cost, such as
@@ -86,7 +84,7 @@
 // Reference caching implementation. Caches references pointing to alloc space(s) for each card.
 class ModUnionTableReferenceCache : public ModUnionTable {
  public:
-  ModUnionTableReferenceCache(Heap* heap) : ModUnionTable(heap) {}
+  explicit ModUnionTableReferenceCache(Heap* heap) : ModUnionTable(heap) {}
   virtual ~ModUnionTableReferenceCache() {}
 
   // Clear and store cards for a space.
@@ -122,7 +120,7 @@
 // Card caching implementation. Keeps track of which cards we cleared and only this information.
 class ModUnionTableCardCache : public ModUnionTable {
  public:
-  ModUnionTableCardCache(Heap* heap) : ModUnionTable(heap) {}
+  explicit ModUnionTableCardCache(Heap* heap) : ModUnionTable(heap) {}
   virtual ~ModUnionTableCardCache() {}
 
   // Clear and store cards for a space.
diff --git a/runtime/gc/accounting/space_bitmap.h b/runtime/gc/accounting/space_bitmap.h
index 32ab440..5a1bfe3 100644
--- a/runtime/gc/accounting/space_bitmap.h
+++ b/runtime/gc/accounting/space_bitmap.h
@@ -244,7 +244,7 @@
     }
   }
 
-  SpaceSetMap(const std::string& name) : name_(name) {}
+  explicit SpaceSetMap(const std::string& name) : name_(name) {}
   ~SpaceSetMap() {}
 
   Objects& GetObjects() {