Add WebGraphicsContext support for readbacks from any framebuffer.

content/renderer/gpu/webgraphicscontext3d_command_buffer_impl.h

Index: content/renderer/gpu/webgraphicscontext3d_command_buffer_impl.h
diff --git a/content/renderer/gpu/webgraphicscontext3d_command_buffer_impl.h b/content/renderer/gpu/webgraphicscontext3d_command_buffer_impl.h
index e538a4564e5863a12ac1aa3ffcb62af8c1a6da2e..39db10a1b811e9d3db4df2b5a0cd2520b21f243a 100644
--- a/content/renderer/gpu/webgraphicscontext3d_command_buffer_impl.h
+++ b/content/renderer/gpu/webgraphicscontext3d_command_buffer_impl.h
@@ -69,6 +69,8 @@ class WebGraphicsContext3DCommandBufferImpl
   virtual void reshape(int width, int height);
 
   virtual bool readBackFramebuffer(unsigned char* pixels, size_t buffer_size);
+  virtual bool readBackFramebuffer(unsigned char* pixels, size_t buffer_size,
+                                   WebGLId framebuffer, int width, int height);
 
   virtual WebGLId getPlatformTextureId();
   virtual void prepareTexture();