Add command buffer support for GL_RGB CHROMIUM image emulation.

gpu/GLES2/extensions/CHROMIUM/CHROMIUM_gpu_memory_buffer_image.txt

Index: gpu/GLES2/extensions/CHROMIUM/CHROMIUM_gpu_memory_buffer_image.txt
diff --git a/gpu/GLES2/extensions/CHROMIUM/CHROMIUM_gpu_memory_buffer_image.txt b/gpu/GLES2/extensions/CHROMIUM/CHROMIUM_gpu_memory_buffer_image.txt
index d185894ade76ac4ea93c0167e11923c7c1e281a1..551e922e901171b821c47efc57d729fcab8346ef 100644
--- a/gpu/GLES2/extensions/CHROMIUM/CHROMIUM_gpu_memory_buffer_image.txt
+++ b/gpu/GLES2/extensions/CHROMIUM/CHROMIUM_gpu_memory_buffer_image.txt
@@ -51,6 +51,12 @@ New Procedures and Functions
 
     INVALID_ENUM is generated if <usage> is not READ_WRTIE_CHROMIUM.
 
+    On OS X, RGB internal format is not supported, and the created
+    GpuMemoryBuffer will always have internal format RGBA. The caller is
+    responsible for handling this appropriately. The only support provided by
+    this extension is that calls to copyTexImage2D and copyTexSubImage2D will
+    perform paramter validation as if the internal format were RGB.

[erikchen, 2016/04/06 23:26:30]

Assuming that everyone is happy with the general idea of this approach, I think
there are cleaner ways of supporting this mechanism in the CHROMIUM image API. 

1. Add an additional parameter to CreateGpuMemoryBufferImageCHROMIUM (bool
RGBEmulation).
2. Add an additional parameter to BindTexImage2DCHROMIUM (bool RGBEmulation).
3. Add a new function MarkImageAsRGBEmulationCHROMIUM(GLuint imageID)
4. [Keep mechanism here].

I'm not sure which one reveman@ will prefer, so I've held off on adding the
plumbing for a new API call. Maybe (3), based on
https://bugs.chromium.org/p/chromium/issues/detail?id=598050#c2, and the desire
to keep this set of APIs as close as possible to EGL image extension APIs?

[reveman, 2016/04/07 13:00:56]

I don't think we should have workarounds as part of extension specs. It should
be limited to the implementation. Why do we need to mark the image as using RGB
emulation? Can that not just be hardcoded as emulated on MacOS if format is
equal to RGB? 

[Ken Russell (switch to Gerrit), 2016/04/07 21:41:14]

It seems reasonable to mention errata in the extension spec. Unfortunately, as
has already been discussed at length on this bug and other CLs, it is
impractical to completely hide the knowledge from users of this extension that
GL_RGB type GLImages are emulated with the GL_RGBA type. Therefore this should
be documented.

+
 Errors
 
     None.