Summary:
Changelog: [internal]
Background executor performs unnecessary operations when second `completeRoot` message from React arrives before first `completeRoot` was finished. This produces unnecessary `ShadowViewMutations`.
Mechanism:
Everytime `completeRoot` is received, before the call is dispatched on the background queue, `completeRootEventCounter_DO_NOT_USE_` is incremented.
Inside `ShadowTree::tryCommit` we check if the value has been incremented to determine if another `completeRoot` is queued.
Reviewed By: JoshuaGross
Differential Revision: D24419160
fbshipit-source-id: 11e19026feca01db6c8981b093a691a6b58a006f