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As a boosted fling may suppress a new scroll sequence in anticipation of a
follow-up fling event, it's important that scrolling properly resume if the
boosted fling terminates during said suppressed scroll. Normally, a synthetic
GestureScrollBegin is used to kickstart the scroll in such cases. However, if
the fling naturally terminates by either overscrolling or ticking beyond its
lifetime, we failed to resume touch scrolling. This led to unexpected cases
where a GestureScrollUpdate would be processed without an antecedent
GestureScrollBegin. 

Address this by always inserting the appropriate synthetic GestureScrollBegin
if the fling terminates for any reason.

BUG=402077

Review URL: https://codereview.chromium.org/473053002

Cr-Commit-Position: refs/heads/master@{#290027}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290027 0039d316-1c4b-4281-b951-d872f2087c98