Bug 1319521 - Properly tear down the LayerTreeState for content. r=gfx?

We were never actually removing the LayerTreeState entry for content layer trees.
In non-webrender codepaths, this happens when a PLayerTransaction belonging to
a CrossProcessCompositorBridge is torn down (see CrossProcessCompositorBridge::
DeallocPLayerTransactionParent). The equivalent of that in the webrender world
is in DeallocPWebRenderBridgeParent, so we should do the same.

gfx/layers/ipc/CrossProcessCompositorBridgeParent.cpp

@@ -228,13 +228,7 @@ CrossProcessCompositorBridgeParent::DeallocPWebRenderBridgeParent(PWebRenderBrid
   MOZ_RELEASE_ASSERT(false);
 #endif
   WebRenderBridgeParent* parent = static_cast<WebRenderBridgeParent*>(aActor);
-  {
-    MonitorAutoLock lock(*sIndirectLayerTreesLock);
-    auto it = sIndirectLayerTrees.find(parent->PipelineId());
-    if (it != sIndirectLayerTrees.end()) {
-      it->second.mWRBridge = nullptr;
-    }
-  }
+  EraseLayerState(parent->PipelineId());
   parent->Release(); // IPDL reference
   return true;
 }