Bug 1548247 - Use aGuid's RenderRoot in RecvStart/StopAutoscroll r=kats

The work for the antecedent patch led me to stumble on a problem where
we were not correctly stopping autoscroll. This was also due to a
renderroot mismatch, which this patch addresses. The call comes through
nsBaseWidget no matter what, it seems, so using mWrRootId.mRenderRoot
seems to be incorrect. I couldn't see a more elegant fix than this.

Differential Revision: https://phabricator.services.mozilla.com/D31863

--HG--
extra : moz-landing-system : lando

gfx/layers/ipc/APZCTreeManagerParent.cpp

@@ -150,7 +150,7 @@ mozilla::ipc::IPCResult APZCTreeManagerParent::RecvStartAutoscroll(
   // sending the child process's layers id).
 
   mUpdater->RunOnControllerThread(
-      UpdaterQueueSelector(mWrRootId),
+      UpdaterQueueSelector(mWrRootId.mLayersId, aGuid.mRenderRoot),
       NewRunnableMethod<SLGuidAndRenderRoot, ScreenPoint>(
           "layers::IAPZCTreeManager::StartAutoscroll", mTreeManager,
           &IAPZCTreeManager::StartAutoscroll, aGuid, aAnchorLocation));
@@ -163,7 +163,7 @@ mozilla::ipc::IPCResult APZCTreeManagerParent::RecvStopAutoscroll(
   // See RecvStartAutoscroll() for why we don't check the layers id.
 
   mUpdater->RunOnControllerThread(
-      UpdaterQueueSelector(mWrRootId),
+      UpdaterQueueSelector(mWrRootId.mLayersId, aGuid.mRenderRoot),
       NewRunnableMethod<SLGuidAndRenderRoot>(
           "layers::IAPZCTreeManager::StopAutoscroll", mTreeManager,
           &IAPZCTreeManager::StopAutoscroll, aGuid));