Expose GpuMemoryBufferId through glGetImageivCHROMIUM.

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h

Index: third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
diff --git a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
index 03286a11be17f589c4acc9b8a77d2a40a79098af..ea1dd22557cccc43e0d41d9515b49f13407c39df 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h
@@ -250,6 +250,11 @@ private:
         DISALLOW_NEW();
         GLuint textureId = 0;
         GLuint imageId = 0;
+
+        // A GpuMemoryBuffer is a concept that the compositor understands. and
+        // is able to operate on. The id is scoped to renderer process.
+        GLint gpuMemoryBufferId = -1;
+
         TextureParameters parameters;
     };