Move setCoveredByChild methods to Tab.

Move setCoveredByChild methods to Tab.

Moves a subclass Tab method up to superclass to eliminate the dependency
on the downstream Tab implementation. Removes the no longer necessary
method from TabDelegate.

BUG=464934
Committed: https://crrev.com/a0bf8e711f92b7567e6ce3d6faba56825d36c13b
Cr-Commit-Position: refs/heads/master@{#319737}

[Maria, 2015-03-09 20:47:45]

mariakhomenko@chromium.org changed reviewers:
+ dfalcantara@chromium.org

[gone, 2015-03-09 20:50:54]

lgtm

[Maria, 2015-03-09 20:51:17]

The CQ bit was checked by mariakhomenko@chromium.org

[commit-bot: I haz the power, 2015-03-09 20:51:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/988323005/1

[commit-bot: I haz the power, 2015-03-09 21:32:17]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-09 21:33:10]

Patchset 1 (id:??) landed as
https://crrev.com/a0bf8e711f92b7567e6ce3d6faba56825d36c13b
Cr-Commit-Position: refs/heads/master@{#319737}