Enable coordinated scrollbars in subframes and overflow scrollers.

Enable coordinated scrollbars in subframes and overflow scrollers.

This removes an obsolete main-frame-only restriction for coordinated scrollbars
on certain platforms (Linux, Windows, and ChromeOS).

The test rebaselines are because GraphicsLayer::m_drawsContent is now false for
scrollbar layers (since the content is drawn by cc instead of Blink).

BUG=535051
Committed: https://crrev.com/b55a5a20a5c8d4d322b05e392a86921048ff3524
Cr-Commit-Position: refs/heads/master@{#351620}

[skobes, 2015-09-29 22:01:01]

Patchset #2 (id:20001) has been deleted

[skobes, 2015-09-29 22:01:20]

Patchset #1 (id:1) has been deleted

[skobes, 2015-09-29 22:02:00]

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

[trchen, 2015-09-30 19:10:34]

lgtm. Cool! If my memory is correct, only Android enabled impl-side scrollbar
everywhere because back in M25 only Android aggressively enabled impl-side
scrolling for everything. We really should enable it whenever impl-side
scrolling is used.

[skobes, 2015-09-30 19:14:03]

The CQ bit was checked by skobes@chromium.org

[commit-bot: I haz the power, 2015-09-30 19:16:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1358173007/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1358173007/40001

[commit-bot: I haz the power, 2015-09-30 19:42:15]

Committed patchset #1 (id:40001)

[commit-bot: I haz the power, 2015-09-30 19:43:29]

Patchset 1 (id:??) landed as
https://crrev.com/b55a5a20a5c8d4d322b05e392a86921048ff3524
Cr-Commit-Position: refs/heads/master@{#351620}