Replace NULL with nullptr

Also fixed some lines that were too long, and a few other minor
details.

Change-Id: I6efba5fb6e03eb5d0a300fddb2a75bf8e2f175cb
diff --git a/runtime/reference_table.cc b/runtime/reference_table.cc
index beba64f..a31d8ac 100644
--- a/runtime/reference_table.cc
+++ b/runtime/reference_table.cc
@@ -40,7 +40,7 @@
 }
 
 void ReferenceTable::Add(mirror::Object* obj) {
-  DCHECK(obj != NULL);
+  DCHECK(obj != nullptr);
   VerifyObject(obj);
   if (entries_.size() >= max_size_) {
     LOG(FATAL) << "ReferenceTable '" << name_ << "' "
@@ -79,8 +79,8 @@
 static void DumpSummaryLine(std::ostream& os, mirror::Object* obj, size_t element_count,
                             int identical, int equiv)
     SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-  if (obj == NULL) {
-    os << "    NULL reference (count=" << equiv << ")\n";
+  if (obj == nullptr) {
+    os << "    null reference (count=" << equiv << ")\n";
     return;
   }
   if (Runtime::Current()->IsClearedJniWeakGlobal(obj)) {