Converts focusmanager from delegate to observer

Converts focusmanager from delegate to observer

I'm going to need more than one object to detect focus changes.

BUG=548424
TEST=covered by tests
R=ben@chromium.org

Committed: https://crrev.com/8db594182d61e2ade6b1fabebd8becd0f1c97f44
Cr-Commit-Position: refs/heads/master@{#361150}

[Ben Goodger (Google), 2015-11-23 18:16:53]

lgtm

[sky, 2015-11-23 18:17:34]

The CQ bit was checked by sky@chromium.org

[commit-bot: I haz the power, 2015-11-23 18:19:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1465143004/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1465143004/20001

[commit-bot: I haz the power, 2015-11-23 19:24:30]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-11-23 19:25:53]

Patchset 2 (id:??) landed as
https://crrev.com/8db594182d61e2ade6b1fabebd8becd0f1c97f44
Cr-Commit-Position: refs/heads/master@{#361150}