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
Committed: https://crrev.com/14ffa6bce4fe48939379ef32c304f02064efad96
Cr-Commit-Position: refs/heads/master@{#307162}

[jdduke (slow), 2014-12-06 04:09:58]

jdduke@chromium.org changed reviewers:
+ aelias@chromium.org

[jdduke (slow), 2014-12-06 04:09:59]

aelias@: PTAL, thanks. Should have noticed this sooner...

[aelias_OOO_until_Jul13, 2014-12-06 04:15:14]

lgtm, thanks.

[jdduke (slow), 2014-12-06 05:56:44]

The CQ bit was checked by jdduke@chromium.org

[commit-bot: I haz the power, 2014-12-06 05:57:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/776943004/20001

[commit-bot: I haz the power, 2014-12-06 07:12:47]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-12-06 07:13:36]

Patchset 2 (id:??) landed as
https://crrev.com/14ffa6bce4fe48939379ef32c304f02064efad96
Cr-Commit-Position: refs/heads/master@{#307162}