Call updateMountedFlag in ShadowTree::tryCommit only if commit will go to native

Summary:
Changelog: [internal]

`updateMountedFlag` needs to be called only when new revision will be mounted. In case a commit is throttled, this wasn't the case. Therefore, moving cancellation of commit so it happens before `updateMountedFlag`. We already cancel commits on that place so it should be safe.

Reviewed By: shergin

Differential Revision: D26049262

fbshipit-source-id: e0ecdd2d8f0cdb09d0c0a07ad3931ce77bcf03cf

ReactCommon/react/renderer/mounting/ShadowTree.cpp

@@ -371,7 +371,8 @@ CommitStatus ShadowTree::tryCommit(
     newRootShadowNode = delegate_.shadowTreeWillCommit(
         *this, oldRootShadowNode, newRootShadowNode);
 
-    if (!newRootShadowNode) {
+    if (!newRootShadowNode ||
+        (commitOptions.shouldCancel && commitOptions.shouldCancel())) {
       return CommitStatus::Cancelled;
     }
 
@@ -392,10 +393,6 @@ CommitStatus ShadowTree::tryCommit(
     currentRevision_ = newRevision;
   }
 
-  if (commitOptions.shouldCancel && commitOptions.shouldCancel()) {
-    return CommitStatus::Cancelled;
-  }
-
   emitLayoutEvents(affectedLayoutableNodes);
 
   if (commitMode == CommitMode::Normal) {