cc: Use the correct internal format for glCopyTexImage2D calls.

content/test/mailbox_output_surface.cc

Index: content/test/mailbox_output_surface.cc
diff --git a/content/test/mailbox_output_surface.cc b/content/test/mailbox_output_surface.cc
index 5edd24b2c872e75e3acf3e5dda8a691aaf398010..4f4dd0d03a51642f9836e588c6465d944bfa924a 100644
--- a/content/test/mailbox_output_surface.cc
+++ b/content/test/mailbox_output_surface.cc
@@ -156,6 +156,10 @@ void MailboxOutputSurface::BindFramebuffer() {
                            current_backing_.texture_id, 0);
 }
 
+uint32_t MailboxOutputSurface::GetFramebufferCopyTextureFormat() {
+  return GLCopyTextureInternalFormat(cc::RGBA_8888);
+}
+
 void MailboxOutputSurface::OnSwapAck(uint32_t output_surface_id,
                                      const cc::CompositorFrameAck& ack) {
   // Ignore message if it's a stale one coming from a different output surface