Make it possible to synthesize a viewport scroll
When targetViewport is true, skip the hit test and always
scroll the viewport layer.
This is to support the WebView#flingScroll API, which has been
implemented to always target at viewport scroll layer.
Accompanying blink change that adds 'targetViewport':
https://codereview.chromium.org/
1187633003/
BUG=493765
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel
Review URL: https://codereview.chromium.org/
1189603004
Cr-Commit-Position: refs/heads/master@{#335750}