Add plumbing for two gpu workarounds, soon to be used by DrawingBuffer.

gpu/command_buffer/common/capabilities.h

Index: gpu/command_buffer/common/capabilities.h
diff --git a/gpu/command_buffer/common/capabilities.h b/gpu/command_buffer/common/capabilities.h
index f4bb19dc43ed26d9a925f2a72865f8d5312bc34e..f7ffb7db9b67a7f4857e8c79ff4a4547fe17d22e 100644
--- a/gpu/command_buffer/common/capabilities.h
+++ b/gpu/command_buffer/common/capabilities.h
@@ -144,6 +144,8 @@ struct GPU_EXPORT Capabilities {
   bool surfaceless;
   bool flips_vertically;
   bool msaa_is_slow;
+  bool disable_webgl_multisampling_color_mask_usage;
+  bool disable_webgl_rgb_multisampling_usage;
 
   // When this parameter is true, a CHROMIUM image created with RGB format will
   // actually have RGBA format. The client is responsible for handling most of