Add support for EXT_image_flush_external extension

BUG=chromium:478339
TEST=Boot to UI

Signed-off-by: Vince Hsu <[email protected]>
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel;tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2109803003
Cr-Commit-Position: refs/heads/master@{#405659}
diff --git a/ui/gl/gl_surface_overlay.cc b/ui/gl/gl_surface_overlay.cc
index 4e5dd056..a8a606b 100644
--- a/ui/gl/gl_surface_overlay.cc
+++ b/ui/gl/gl_surface_overlay.cc
@@ -33,4 +33,8 @@
                                       bounds_rect_, crop_rect_);
 }
 
+void GLSurfaceOverlay::Flush() const {
+  return image_->Flush();
+}
+
 }  // namespace gl