Add "targetViewport" to ScrollBegin and FlingStart
Add "targetViewport" to ScrollBegin and FlingStart

If the bit is true, the scroll will be targeted to the viewport scroll
layer. If false, the scroll may or may not be targeted to the viewport
scroll layer, depending on the hit test.

BUG=493765

Review URL: https://codereview.chromium.org/1187633003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@197411 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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