harden requirements on SkBitmapCache

harden requirements on SkBitmapCache

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/14b6aba0997a8b463eae824d770fc5ad1ab89617

[reed1, 2014-08-29 14:10:21]

reed@google.com changed reviewers:
+ humper@google.com

[reed1, 2014-08-29 15:38:08]

ptal

[reed1, 2014-08-29 16:23:58]

The CQ bit was checked by reed@google.com

[commit-bot: I haz the power, 2014-08-29 16:24:55]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/reed@google.com/521433002/20001

[commit-bot: I haz the power, 2014-08-29 16:24:55]

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 a valid reviewer till 2014-08-29 22:24 UTC

[humper, 2014-08-29 17:23:08]

On 2014/08/29 16:24:55, I haz the power (commit-bot) wrote:
> 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 a valid reviewer till 2014-08-29 22:24 UTC

LGTM

[commit-bot: I haz the power, 2014-08-29 17:25:29]

Committed patchset #2 (id:20001) as 14b6aba0997a8b463eae824d770fc5ad1ab89617