Fix bug in aggressive layer cache purging

Fix bug in aggressive layer cache purging

A picture may possess many layers that get placed in one plot of the atlas. In this case we can only remove the plot from the plotUsage tracking structure when all the layers belonging to the picture in that plot have been removed.

Committed: https://skia.googlesource.com/skia/+/225a627ccb15f41efa40750f4244283ef7e8fbf0

[robertphillips, 2014-10-30 17:57:11]

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

[robertphillips, 2014-10-30 18:03:31]

The CQ bit was checked by robertphillips@google.com

[commit-bot: I haz the power, 2014-10-30 18:03:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/654463004/1

[commit-bot: I haz the power, 2014-10-30 18:03:57]

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 2014-10-31 00:03 UTC

[jvanverth1, 2014-10-30 18:12:32]

lgtm

[commit-bot: I haz the power, 2014-10-30 18:39:24]

Committed patchset #1 (id:1) as 225a627ccb15f41efa40750f4244283ef7e8fbf0