Dispatch top content inset changes immediately if synchronously waiting for view...

Dispatch top content inset changes immediately if synchronously waiting for view state changes
https://bugs.webkit.org/show_bug.cgi?id=134942
<rdar://problem/17666800>

Reviewed by Simon Fraser.

* UIProcess/API/mac/WKView.mm:
(-[WKView endDeferringViewInWindowChanges]):
(-[WKView endDeferringViewInWindowChangesSync]):
(-[WKView _dispatchSetTopContentInset]):
(-[WKView _setTopContentInset:]):
Send top content inset changes immediately before sync-waiting for new tiles from the Web Process.
This will ensure that the incoming contents have the right top content inset, and we don't
flash between the wrong inset and the right one.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@171117 268f45cc-cd09-0410-ab3c-d52691b4dbfc