Respect the smooth scrolling setting in LayerScrollableArea.

Reland: Respect the smooth scrolling setting in LayerScrollableArea.

This relands https://blink.lc/blink/commit/?id=5668429, which was reverted in
https://blink.lc/blink/commit/?id=a1303a8.

The Mac trackpad regression is addressed by
https://blink.lc/blink/commit/?id=021397d.

BUG=575, 417782
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=190675

[skobes, 2015-01-29 01:23:05]

Patchset #1 (id:1) has been deleted

[skobes, 2015-01-30 00:33:58]

Patchset #1 (id:20001) has been deleted

[skobes, 2015-01-31 01:04:57]

skobes@chromium.org changed reviewers:
+ ajuma@chromium.org

[ajuma, 2015-02-02 14:43:03]

lgtm

[skobes, 2015-02-04 23:54:08]

There's something wrong with the animations here that I need to look into before
landing this.  For some reason they're running much slower than the equivalent
frame-level scrolls.

[skobes, 2015-02-10 01:10:49]

Patchset #3 (id:80001) has been deleted

[skobes, 2015-02-10 03:46:28]

skobes@chromium.org changed reviewers:
+ japhet@chromium.org

[skobes, 2015-02-10 03:46:29]

+japhet for Source/web OWNERS

The slow animation issue is fixed by http://crrev.com/905463007 and I have
updated the test.

[Nate Chapin, 2015-02-11 17:52:42]

lgtm

[skobes, 2015-02-11 21:10:59]

The CQ bit was checked by skobes@chromium.org

[commit-bot: I haz the power, 2015-02-11 21:11:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/882813005/120001

[commit-bot: I haz the power, 2015-02-11 22:22:56]

Committed patchset #4 (id:120001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=190002

[ccameron, 2015-02-18 21:54:24]

A revert of this CL (patchset #4 id:120001) has been created in
https://codereview.chromium.org/933823004/ by ccameron@chromium.org.

The reason for reverting is: Regresses scrolling on Mac:
http://crbug.com/459739.

[skobes, 2015-02-20 23:48:44]

New patchsets have been uploaded after l-g-t-m from japhet@chromium.org

[skobes, 2015-02-23 18:24:34]

Mac regression fixed in http://crrev.com/941843002, relanding.

[skobes, 2015-02-23 18:24:38]

The CQ bit was checked by skobes@chromium.org

[commit-bot: I haz the power, 2015-02-23 18:25:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/882813005/140001

[commit-bot: I haz the power, 2015-02-23 18:37:15]

Committed patchset #5 (id:140001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=190675