Scrolling does not work inside nested frames
Created attachment 236319 [details] Patch
<rdar://problem/17959896>
https://trac.webkit.org/r172365
A test landed with this patch is super flaky on Mac WK2: http://webkit-test-results.appspot.com/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=platform%2Fmac-wk2%2Ftiled-drawing%2Fscrolling%2Fframes%2Fframeset-nested-frame-scrollability.html
Now tracked in bug 136554.