Bug 1655584 - Flush rendering for popup window configuration changes with GTK. r=sotaro

This backs out part of bug 1656211 which turned out to be insufficient.
The invalidate rendered frame transaction races with the initial frame
rendering of the popup. If it comes in too soon, we will only draw the
frame once, and the frame corruption remains. This patch makes us flush
the rendering pipeline to ensure we get two separate generate frame
events.

Differential Revision: https://phabricator.services.mozilla.com/D96157

gfx/layers/wr/WebRenderLayerManager.cpp

@@ -709,10 +709,6 @@ void WebRenderLayerManager::ScheduleComposite() {
   WrBridge()->SendScheduleComposite();
 }
 
-void WebRenderLayerManager::ForceComposite() {
-  WrBridge()->SendForceComposite();
-}
-
 void WebRenderLayerManager::SetRoot(Layer* aLayer) {
   // This should never get called
   MOZ_ASSERT(false);