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scrolling.

When wheel scroll latching is enabled, GSB occurs only once in the
beginning of a scroll sequence. So, it is possible to need to switch
to the main thread scroll handling in the middle of a scroll scroll
sequence (e.g. due to a change in main thread scrolling reasons).
To have a complete scroll sequence on main thread side and do the scroll
chain computation only while handling GSB, this cl sends a GSB to the main thread before sending the GSU that is not handled in the impl thread
side.

BUG=526463
TEST=AnimateInput/InputHandlerProxyTest.GestureScrollHandlingSwitchedToMainThread/*

Review-Url: https://codereview.chromium.org/2854683002
Cr-Commit-Position: refs/heads/master@{#471006}