Don't clobber active page scale animation with null pointer.

Don't clobber active page scale animation with null pointer.

r297913 introduced the bug that if another activation happens
during a page scale animation, it's clobbered by a null pointer.
Only copy into LTHI::page_scale_animation_ when the LTI has an instance
of it.

NOTRY=true
BUG=419450
Committed: https://crrev.com/74c39357ba160b985378a5867fac496ea49f23ad
Cr-Commit-Position: refs/heads/master@{#299421}

[aelias_OOO_until_Jul13, 2014-10-14 00:32:43]

aelias@chromium.org changed reviewers:
+ bokan@chromium.org

[aelias_OOO_until_Jul13, 2014-10-14 00:32:49]

Hi bokan@, PTAL.

[bokan, 2014-10-14 02:12:18]

lgtm, thanks for tracking these down.

[aelias_OOO_until_Jul13, 2014-10-14 03:43:59]

The CQ bit was checked by aelias@chromium.org

[commit-bot: I haz the power, 2014-10-14 03:45:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/650353003/1

[commit-bot: I haz the power, 2014-10-14 03:48:22]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-14 03:49:12]

Patchset 1 (id:??) landed as
https://crrev.com/74c39357ba160b985378a5867fac496ea49f23ad
Cr-Commit-Position: refs/heads/master@{#299421}