Prevent a negative duration animation

Prevent a negative duration animation

Haven't been able to repro this locally, but it's apparently possible for a
calculation to return a negative value for an animation duration, resulting in
a crash.

BUG=481031
Committed: https://crrev.com/930d5e7d4878c1adf0d7b282b24324912778dc2a
Cr-Commit-Position: refs/heads/master@{#326871}

[gone, 2015-04-24 19:49:15]

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

[aurimas (slooooooooow), 2015-04-24 20:40:01]

lgtm

[gone, 2015-04-24 20:47:59]

The CQ bit was checked by dfalcantara@chromium.org

[commit-bot: I haz the power, 2015-04-24 20:48:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1108653002/1

[commit-bot: I haz the power, 2015-04-24 20:53:42]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-24 20:54:41]

Patchset 1 (id:??) landed as
https://crrev.com/930d5e7d4878c1adf0d7b282b24324912778dc2a
Cr-Commit-Position: refs/heads/master@{#326871}