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/code_generator_mips.cc b/compiler/optimizing/code_generator_mips.cc
index 476e8ab..aed334b 100644
--- a/compiler/optimizing/code_generator_mips.cc
+++ b/compiler/optimizing/code_generator_mips.cc
@@ -7454,7 +7454,7 @@
   // Given the numeric representation, it's enough to check the low bit of the
   // rb_state. We do that by shifting the bit into the sign bit (31) and
   // performing a branch on less than zero.
-  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");
   static_assert(LockWord::kReadBarrierStateSize == 1, "Expecting 1-bit read barrier state size");
   __ Sll(temp_reg, temp_reg, 31 - LockWord::kReadBarrierStateShift);