Bug 1470090 - Fix LayerObserverEpoch handling r=nical

dom/ipc/TabChild.cpp

@@ -3228,6 +3228,9 @@ TabChild::ReinitRendering()
   gfx::VRManagerChild::IdentifyTextureHost(mTextureFactoryIdentifier);
 
   InitAPZState();
+  RefPtr<LayerManager> lm = mPuppetWidget->GetLayerManager();
+  MOZ_ASSERT(lm);
+  lm->SetLayerObserverEpoch(mLayerObserverEpoch);
 
   nsCOMPtr<nsIDocument> doc(GetDocument());
   doc->NotifyLayerManagerRecreated();

gfx/layers/wr/WebRenderBridgeParent.cpp

@@ -682,10 +682,6 @@ WebRenderBridgeParent::RecvSetDisplayList(const gfx::IntSize& aSize,
       // build is done, so we don't need to do it here.
       ScheduleGenerateFrame();
     }
-
-    if (ShouldParentObserveEpoch()) {
-      mCompositorBridge->ObserveLayerUpdate(GetLayersId(), GetChildLayerObserverEpoch(), true);
-    }
   }
 
   HoldPendingTransactionId(wrEpoch, aTransactionId, aTxnStartTime, aFwdTime);
@@ -697,6 +693,10 @@ WebRenderBridgeParent::RecvSetDisplayList(const gfx::IntSize& aSize,
     mCompositorBridge->DidComposite(GetLayersId(), now, now);
   }
 
+  if (ShouldParentObserveEpoch()) {
+    mCompositorBridge->ObserveLayerUpdate(GetLayersId(), GetChildLayerObserverEpoch(), true);
+  }
+
   wr::IpcResourceUpdateQueue::ReleaseShmems(this, aSmallShmems);
   wr::IpcResourceUpdateQueue::ReleaseShmems(this, aLargeShmems);
   return IPC_OK();
@@ -771,6 +771,10 @@ WebRenderBridgeParent::RecvEmptyTransaction(const FocusTarget& aFocusTarget,
     mCompositorBridge->DidComposite(GetLayersId(), now, now);
   }
 
+  if (ShouldParentObserveEpoch()) {
+    mCompositorBridge->ObserveLayerUpdate(GetLayersId(), GetChildLayerObserverEpoch(), true);
+  }
+
   return IPC_OK();
 }