Bug 1421196 - Set default draw target to basic layer manager so that we won't have different backend. r=kats

MozReview-Commit-ID: IWzzPvAcwQ5

--HG--
extra : rebase_source : 28359fdfca721b29a93de5d0cc88d1c5ecc59a06

gfx/layers/wr/WebRenderCommandBuilder.cpp

@@ -355,6 +355,7 @@ PaintByLayer(nsDisplayItem* aItem,
   layerBuilder->Init(aDisplayListBuilder, aManager, nullptr, true);
   layerBuilder->DidBeginRetainedLayerTransaction(aManager);
 
+  aManager->SetDefaultTarget(aContext);
   aManager->BeginTransactionWithTarget(aContext);
   bool isInvalidated = false;
 
@@ -393,6 +394,7 @@ PaintByLayer(nsDisplayItem* aItem,
   }
 
   aManager->SetTarget(nullptr);
+  aManager->SetDefaultTarget(nullptr);
 
   return isInvalidated;
 }