Add additional clean up code in MPD path

Add additional clean up code in MPD path

This addresses an edge case where a picture only has one layer which is atlasable but doesn't make it into the atlas. In this case asserts can fire since there is no atlased layer to clean up the tracking picture object.

Committed: https://skia.googlesource.com/skia/+/bc0c4bd9dff5c7be1a0845ef0d5cc3f4faaef5f3

[robertphillips, 2014-12-01 17:41:58]

robertphillips@google.com changed reviewers:
+ bsalomon@google.com

[robertphillips, 2014-12-01 19:00:01]

The CQ bit was checked by robertphillips@google.com

[commit-bot: I haz the power, 2014-12-01 19:00:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/764393002/1

[commit-bot: I haz the power, 2014-12-01 19:00:15]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2014-12-02 01:00 UTC

[robertphillips, 2014-12-01 19:01:37]

The CQ bit was unchecked by robertphillips@google.com

[bsalomon, 2014-12-01 19:30:14]

lgtm

[robertphillips, 2014-12-01 19:38:54]

The CQ bit was checked by robertphillips@google.com

[commit-bot: I haz the power, 2014-12-01 19:39:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/764393002/1

[commit-bot: I haz the power, 2014-12-01 19:40:02]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/bc0c4bd9dff5c7be1a0845ef0d5cc3f4faaef5f3