Bug 1143575. Android's screenshotting code should invalidate the LayerManagerComposite to ensure composition will actually happen. r=nical

There is some ambiguity about whether ScheduleComposite will necessarily
trigger a composite all the way to nsWindow::DrawWindowUnderlay. Android
robocop tests assume it will, because they rely on DrawWindowOverlay
being called so they can take a screenshot and make progress,
but this is a very fragile assumption. They also rely on the entire
window being painted, which is also a fragile assumption.

This patch improves the situation by explicitly invalidating the current
window area when Android Java code needs to trigger a composite. This avoids
regressions from future patches in this series which make composition bail
out when there is nothing invalid.

The resulting setup is still a bit fragile for my taste but I'm not sure
what the ideal solution would be.

dom/plugins/base/nsNPAPIPluginInstance.cpp

@@ -978,7 +978,7 @@ already_AddRefed<AndroidSurfaceTexture> nsNPAPIPluginInstance::CreateSurfaceText
 void nsNPAPIPluginInstance::OnSurfaceTextureFrameAvailable()
 {
   if (mRunning == RUNNING && mOwner)
-    AndroidBridge::Bridge()->ScheduleComposite();
+    AndroidBridge::Bridge()->InvalidateAndScheduleComposite();
 }
 
 void* nsNPAPIPluginInstance::AcquireContentWindow()