Rename art::ReadBarrier::WhiteState as art::ReadBarrier::NonGrayState.

The read barrier state recorded in object's lockword used to be a
three-state value (white/gray/black), which was turned into a
two-state value (white/gray), where the "black" state was conceptually
merged into the "white" state. This change renames the "white" state
as "non-gray" and adjusts corresponding comments.

Test: art/test.py
Change-Id: I2a17ed15651bdbbe99270c1b81b4d78a1c2c132b
diff --git a/runtime/gc/reference_queue.cc b/runtime/gc/reference_queue.cc
index 321d22a..e25e279 100644
--- a/runtime/gc/reference_queue.cc
+++ b/runtime/gc/reference_queue.cc
@@ -76,19 +76,19 @@
   Heap* heap = Runtime::Current()->GetHeap();
   if (kUseBakerOrBrooksReadBarrier && heap->CurrentCollectorType() == kCollectorTypeCC &&
       heap->ConcurrentCopyingCollector()->IsActive()) {
-    // Change the gray ptr we left in ConcurrentCopying::ProcessMarkStackRef() to white.
+    // Change the gray ptr we left in ConcurrentCopying::ProcessMarkStackRef() to non-gray.
     // We check IsActive() above because we don't want to do this when the zygote compaction
     // collector (SemiSpace) is running.
     CHECK(ref != nullptr);
     collector::ConcurrentCopying* concurrent_copying = heap->ConcurrentCopyingCollector();
     uint32_t rb_state = ref->GetReadBarrierState();
     if (rb_state == ReadBarrier::GrayState()) {
-      ref->AtomicSetReadBarrierState(ReadBarrier::GrayState(), ReadBarrier::WhiteState());
-      CHECK_EQ(ref->GetReadBarrierState(), ReadBarrier::WhiteState());
+      ref->AtomicSetReadBarrierState(ReadBarrier::GrayState(), ReadBarrier::NonGrayState());
+      CHECK_EQ(ref->GetReadBarrierState(), ReadBarrier::NonGrayState());
     } else {
-      // In ConcurrentCopying::ProcessMarkStackRef() we may leave a white reference in the queue and
-      // find it here, which is OK.
-      CHECK_EQ(rb_state, ReadBarrier::WhiteState()) << "ref=" << ref << " rb_state=" << rb_state;
+      // In ConcurrentCopying::ProcessMarkStackRef() we may leave a non-gray reference in the queue
+      // and find it here, which is OK.
+      CHECK_EQ(rb_state, ReadBarrier::NonGrayState()) << "ref=" << ref << " rb_state=" << rb_state;
       ObjPtr<mirror::Object> referent = ref->GetReferent<kWithoutReadBarrier>();
       // The referent could be null if it's cleared by a mutator (Reference.clear()).
       if (referent != nullptr) {