Bug 1458414 - Call DiscardCompositorAnimations() after BuildWebRenderCommands(). r=kats

Since bug 1457448 we can clear discarded animations on the same transaction where we send
animations to the compositor.

MozReview-Commit-ID: 4BYsCgA98S0

gfx/layers/wr/WebRenderLayerManager.cpp

@@ -264,7 +264,6 @@ WebRenderLayerManager::EndTransactionWithoutLayer(nsDisplayList* aDisplayList,
   mAnimationReadyTime = TimeStamp::Now();
 
   WrBridge()->BeginTransaction();
-  DiscardCompositorAnimations();
 
   LayoutDeviceIntSize size = mWidget->GetClientSize();
   wr::LayoutSize contentSize { (float)size.width, (float)size.height };
@@ -279,6 +278,8 @@ WebRenderLayerManager::EndTransactionWithoutLayer(nsDisplayList* aDisplayList,
                                                   contentSize,
                                                   aFilters);
 
+  DiscardCompositorAnimations();
+
   mWidget->AddWindowOverlayWebRenderCommands(WrBridge(), builder, resourceUpdates);
   mWindowOverlayChanged = false;