[embedlite] Only widget hosting window can create compositor. Fixes J…

…B#35134

Child widgets created by EmbedLitePuppetWidget cannot end up
creating new compositor when requesting LayerManager (GetLayerManager).

On possible backtrace:
1) EmbedLitePuppetWidget::Invalidate
2) nsView::WillPaintWindow
3) nsViewManager::WillPaintWindow
4) EmbedLitePuppetWidget::GetLayerManager
5) EmbedLitePuppetWidget::CreateCompositor

See also https://bugs.merproject.org/show_bug.cgi?id=1603

embedding/embedlite/embedshared/EmbedLitePuppetWidget.cpp

@@ -635,7 +635,10 @@ EmbedLitePuppetWidget::GetLayerManager(PLayerTransactionChild* aShadowManager,
 
   mUseLayersAcceleration = ComputeShouldAccelerate(mUseLayersAcceleration);
 
-  if (ShouldUseOffMainThreadCompositing()) {
+  // TODO : We should really split this into Android/Gonk like nsWindow and separate PuppetWidget
+  // Only Widget hosting window can create compositor.
+  // Bug: https://bugs.merproject.org/show_bug.cgi?id=1603
+  if (mWindow && ShouldUseOffMainThreadCompositing()) {
     CreateCompositor();
     if (mLayerManager) {
       return mLayerManager;