Fix google-explicit-constructor warnings in art.

* Add explicit keyword to conversion constructors,
  or NOLINT for implicit converters.
Bug: 28341362
Test: build with WITH_TIDY=1

Change-Id: I1e1ee2661812944904fedadeff97b620506db47d
diff --git a/runtime/gc/collector/concurrent_copying.h b/runtime/gc/collector/concurrent_copying.h
index 1ef0aea..53473f0 100644
--- a/runtime/gc/collector/concurrent_copying.h
+++ b/runtime/gc/collector/concurrent_copying.h
@@ -61,9 +61,9 @@
   // pages.
   static constexpr bool kGrayDirtyImmuneObjects = true;
 
-  ConcurrentCopying(Heap* heap,
-                    const std::string& name_prefix = "",
-                    bool measure_read_barrier_slow_path = false);
+  explicit ConcurrentCopying(Heap* heap,
+                             const std::string& name_prefix = "",
+                             bool measure_read_barrier_slow_path = false);
   ~ConcurrentCopying();
 
   virtual void RunPhases() OVERRIDE
diff --git a/runtime/gc/collector/garbage_collector.h b/runtime/gc/collector/garbage_collector.h
index 6afe876..4ffa254 100644
--- a/runtime/gc/collector/garbage_collector.h
+++ b/runtime/gc/collector/garbage_collector.h
@@ -44,7 +44,7 @@
 namespace collector {
 
 struct ObjectBytePair {
-  ObjectBytePair(uint64_t num_objects = 0, int64_t num_bytes = 0)
+  explicit ObjectBytePair(uint64_t num_objects = 0, int64_t num_bytes = 0)
       : objects(num_objects), bytes(num_bytes) {}
   void Add(const ObjectBytePair& other) {
     objects += other.objects;