cc: Use property tree index accessors in Layer::On<Property>Animated

cc: Use property tree index accessors in Layer::On<Property>Animated

This fixes two places where we directly access property tree indices
rather than using their accessors.

BUG=489725
Committed: https://crrev.com/dc1cefdb7494c984074c7bf46b3334c3390f92ee
Cr-Commit-Position: refs/heads/master@{#330818}

[ajuma, 2015-05-20 20:43:54]

ajuma@chromium.org changed reviewers:
+ vollick@chromium.org

[Ian Vollick, 2015-05-20 20:44:57]

On 2015/05/20 at 20:44:32, ajuma wrote:
> 

lgtm.

[ajuma, 2015-05-20 20:46:00]

The CQ bit was checked by ajuma@chromium.org

[commit-bot: I haz the power, 2015-05-20 20:47:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1144343002/1

[commit-bot: I haz the power, 2015-05-20 22:29:12]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-20 22:31:07]

Patchset 1 (id:??) landed as
https://crrev.com/dc1cefdb7494c984074c7bf46b3334c3390f92ee
Cr-Commit-Position: refs/heads/master@{#330818}