Properly handle viewport-flings after a bubbling scroll

Properly handle viewport-flings after a bubbling scroll

A compositor-driven fling will lock to the first layer that scrolls
after the fling starts. This can often be a different scrolling layer
than was first scrolled, e.g., when a scrolling layer reaches its limit
and the user then initiates a fling.

A recent change (r304898) inadvertently prevents the fling from locking
to the viewport layer after a bubbling touch scroll. Fix this by
ensuring the scrolling layer is always set after the fling's first
scroll animation update.

BUG=439698

Review URL: https://codereview.chromium.org/776943004

Cr-Commit-Position: refs/heads/master@{#307162}