Remove support for custom overscroll background

Remove support for custom overscroll background

Mac was the only platform to do this, and more recent versions
of the OS do not support it anymore. Callers to these functions
have been removed from the Blink side.

BUG=248742
Committed: https://crrev.com/cbcd8ee26310f2e9a3be4395362a9be1c490c897
Cr-Commit-Position: refs/heads/master@{#312529}

[ccameron, 2015-01-06 19:23:20]

ccameron@chromium.org changed reviewers:
+ danakj@chromium.org, jam@chromium.org

[ccameron, 2015-01-06 19:23:20]

Adding danakj for cc/ and jam for content+mojo OWNER stamp.

[danakj, 2015-01-06 19:43:55]

LGTM

[jam, 2015-01-12 22:27:34]

lgtm

[ccameron, 2015-01-12 22:31:16]

Thanks!

[ccameron, 2015-01-12 22:31:19]

The CQ bit was checked by ccameron@chromium.org

[commit-bot: I haz the power, 2015-01-12 22:32:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/838603002/1

[commit-bot: I haz the power, 2015-01-12 22:42:37]

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

[commit-bot: I haz the power, 2015-01-12 22:42:38]

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

[ccameron, 2015-01-21 23:45:16]

The CQ bit was checked by ccameron@chromium.org

[ccameron, 2015-01-21 23:45:16]

The CQ bit was unchecked by ccameron@chromium.org

[ccameron, 2015-01-21 23:45:19]

The CQ bit was checked by ccameron@chromium.org

[commit-bot: I haz the power, 2015-01-21 23:46:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/838603002/20001

[ccameron, 2015-01-22 00:12:42]

The CQ bit was checked by ccameron@chromium.org

[commit-bot: I haz the power, 2015-01-22 00:13:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/838603002/60001

[commit-bot: I haz the power, 2015-01-22 02:20:55]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-01-22 02:21:35]

Patchset 4 (id:??) landed as
https://crrev.com/cbcd8ee26310f2e9a3be4395362a9be1c490c897
Cr-Commit-Position: refs/heads/master@{#312529}