Change private API names and return type in FocusController

Change private API names and return type in FocusController

This renames private API names in FocusController, to return Element
instead of Node.

No functional change is included.

This is a part of code cleanup that focused node should be Element.
This is a second half of https://codereview.chromium.org/1177373002/

BUG=499667
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197425

[kochi, 2015-06-16 06:49:22]

The CQ bit was checked by kochi@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-06-16 06:49:29]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1180843005/1

[kochi, 2015-06-16 06:53:50]

kochi@chromium.org changed reviewers:
+ hayato@chromium.org

[kochi, 2015-06-16 07:05:18]

The CQ bit was checked by kochi@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-06-16 07:05:29]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1180843005/20001

[kochi, 2015-06-16 07:07:24]

PTAL

[commit-bot: I haz the power, 2015-06-16 08:33:24]

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

[commit-bot: I haz the power, 2015-06-16 08:33:25]

Dry run: This issue passed the CQ dry run.

[hayato, 2015-06-19 01:23:33]

lgtm

[kochi, 2015-06-19 02:04:08]

The CQ bit was checked by kochi@chromium.org

[commit-bot: I haz the power, 2015-06-19 02:04:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1180843005/20001

[commit-bot: I haz the power, 2015-06-19 03:22:05]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197425