Remove unused paintScrollbarThumb impl.

Remove unused Mac implementation of paintScrollbarThumb.

WebThemeEngine::paintScrollbarThumb is no longer called by Blink since
http://crrev.com/544913002.

BUG=327471
Committed: https://crrev.com/ec857a14cb939730c846a2bfe6ae508a20b83eb2
Cr-Commit-Position: refs/heads/master@{#299237}

[skobes, 2014-10-10 22:57:04]

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

[skobes, 2014-10-10 22:58:42]

Oops... hold off on reviewing this.

[skobes, 2014-10-10 23:11:01]

skobes@chromium.org changed reviewers:
- avi@chromium.org

[skobes, 2014-10-10 23:52:42]

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

[skobes, 2014-10-10 23:52:43]

Ok this is ready for review.

[Avi (use Gerrit), 2014-10-11 03:08:14]

lgtm

rrreeeedddd

[skobes, 2014-10-11 03:45:50]

The CQ bit was checked by skobes@chromium.org

[commit-bot: I haz the power, 2014-10-11 03:46:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/647223002/210001

[commit-bot: I haz the power, 2014-10-11 04:55:33]

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

[commit-bot: I haz the power, 2014-10-11 04:55:34]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_swarming on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[skobes, 2014-10-11 04:57:57]

The CQ bit was checked by skobes@chromium.org

[commit-bot: I haz the power, 2014-10-11 04:58:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/647223002/210001

[commit-bot: I haz the power, 2014-10-11 06:05:08]

Committed patchset #2 (id:210001)

[commit-bot: I haz the power, 2014-10-11 06:05:52]

Patchset 2 (id:??) landed as
https://crrev.com/ec857a14cb939730c846a2bfe6ae508a20b83eb2
Cr-Commit-Position: refs/heads/master@{#299237}