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/compiler/optimizing/intrinsics_arm_vixl.cc b/compiler/optimizing/intrinsics_arm_vixl.cc
index b920750..53b0aa2 100644
--- a/compiler/optimizing/intrinsics_arm_vixl.cc
+++ b/compiler/optimizing/intrinsics_arm_vixl.cc
@@ -2205,7 +2205,7 @@
       // Given the numeric representation, it's enough to check the low bit of the
       // rb_state. We do that by shifting the bit out of the lock word with LSRS
       // which can be a 16-bit instruction unlike the TST immediate.
-      static_assert(ReadBarrier::WhiteState() == 0, "Expecting white to have value 0");
+      static_assert(ReadBarrier::NonGrayState() == 0, "Expecting non-gray to have value 0");
       static_assert(ReadBarrier::GrayState() == 1, "Expecting gray to have value 1");
       __ Lsrs(temp2, temp2, LockWord::kReadBarrierStateShift + 1);
       // Carry flag is the last bit shifted out by LSRS.