gpu: consolidate id_type_traits.h and gpu_command_buffer_traits*

gpu/command_buffer/common/id_type.h

Index: gpu/command_buffer/common/id_type.h
diff --git a/gpu/command_buffer/common/id_type.h b/gpu/command_buffer/common/id_type.h
index 197736451a1fa85607839413db9a2c31d9fd5408..0d009c61867c2dac2be6861e1a75c460c8cdc2a7 100644
--- a/gpu/command_buffer/common/id_type.h
+++ b/gpu/command_buffer/common/id_type.h
@@ -35,7 +35,7 @@
 // - it can be used as an argument to DCHECK_EQ or streamed to LOG(ERROR);
 // - it has the same memory footprint and runtime overhead as int32_t;
 // - it can be copied by memcpy.
-// - it can be used in IPC messages (for now via gpu/ipc/id_type_traits.h)
+// - it can be used in IPC messages.
 //
 // IdType32<Foo> has the following differences from a bare int32_t:
 // - it forces coercions to go through GetUnsafeValue and FromUnsafeValue;