Add support for EXT_image_flush_external extension

ui/gl/gl_image.h

Index: ui/gl/gl_image.h
diff --git a/ui/gl/gl_image.h b/ui/gl/gl_image.h
index 8a7a21203f9404196d8a10caae8afc27ca042ae5..bb2cd5ddcd7c04fb996ef95827fce7f4f2ec5c80 100644
--- a/ui/gl/gl_image.h
+++ b/ui/gl/gl_image.h
@@ -92,6 +92,9 @@ class GL_EXPORT GLImage : public base::RefCounted<GLImage> {
   };
   virtual Type GetType() const;
 
+  // Flush any preceding rendering for the image.
+  virtual void Flush() = 0;

[reveman, 2016/07/11 18:59:06]

nit: EmulatingRGB and GetType are temporary workarounds but I think this Flush
function is part of the core API. Please move it below ScheduleOverlayPlane and
maintain the sorting order where implemented.

+
  protected:
   virtual ~GLImage() {}