Fix possible bad cast in Tab

It was possible to think we had set tab_animation_ to a MultiAnimation
when we hadn't. To avoid errors like this I'm splitting tab_animation_
into two distinct fields. Less possibility of error this way.

BUG=387371
TEST=none

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=282455

[Ben Goodger (Google), 2014-07-10 15:49:28]

lgtm

[sky, 2014-07-10 16:03:04]

The CQ bit was checked by sky@chromium.org

[commit-bot: I haz the power, 2014-07-10 16:05:09]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sky@chromium.org/382693002/40001

[sky, 2014-07-10 19:27:42]

The CQ bit was checked by sky@chromium.org

[commit-bot: I haz the power, 2014-07-10 19:29:15]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sky@chromium.org/382693002/60001

[commit-bot: I haz the power, 2014-07-10 21:30:54]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  android_dbg_triggered_tests on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_dbg_triggered...)

[commit-bot: I haz the power, 2014-07-10 21:55:47]

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

[commit-bot: I haz the power, 2014-07-10 21:55:48]

Try jobs failed on following builders:
  android_dbg_triggered_tests on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_dbg_triggered...)

[sky, 2014-07-10 23:12:28]

The CQ bit was checked by sky@chromium.org

[commit-bot: I haz the power, 2014-07-10 23:14:11]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sky@chromium.org/382693002/60001

[commit-bot: I haz the power, 2014-07-10 23:15:55]

Change committed as 282455