Early terminate flings when scrolling impossible

Early terminate flings when scrolling impossible

Previously, flings would continue animating until they were either interrupted,
or had reached the end of their respective curve.  Instead, allow the fling to
terminate early if they hit their limit, given the result of
|WebGestureCurveTarget::scrollBy|. This saves power on sites where fling scrolls
have little affordance.

Also simplify WebGestureCurveTarget by consolidating scroll delta and velocity
into a single method.

Corresponding Chromium patch:
https://codereview.chromium.org/136173004/

Final Blink cleanup patch:
https://codereview.chromium.org/217893004

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170406

[aelias_OOO_until_Jul13, 2014-03-28 23:57:50]

lgtm.  (Just think about how the Blink rolls are going to work when you change
this API.)

[jdduke (slow), 2014-03-29 01:11:00]

jamesr@: Owner review for public/platform? Thanks.

[jdduke (slow), 2014-03-29 01:14:41]

Oops, should have paid attention to jamesr@ OOO label...

abarth@: Owner review for public/platform? Thanks.

[abarth-chromium, 2014-03-29 02:15:18]

lgtm

[jdduke (slow), 2014-03-29 02:55:04]

The CQ bit was checked by jdduke@chromium.org

[commit-bot: I haz the power, 2014-03-29 02:55:09]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jdduke@chromium.org/139493003/60001

[commit-bot: I haz the power, 2014-03-29 03:05:22]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-29 03:05:23]

Try jobs failed on following builders:
  tryserver.blink on win_blink_compile_dbg

[jdduke (slow), 2014-03-29 03:07:58]

The CQ bit was checked by jdduke@chromium.org

[commit-bot: I haz the power, 2014-03-29 03:08:00]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jdduke@chromium.org/139493003/60001

[commit-bot: I haz the power, 2014-03-29 04:04:44]

Change committed as 170406