Change SkResourceCache to take a Visitor inside its find().

Change SkResourceCache to take a Visitor inside its find().

This simplifies the API/contract, in that there are not any exposed
lock/unlock scopes.


patch from issue 572573002

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/dee6a8e67db39fcbde2b3bb09be1d088ebb9db8a

[reed1, 2014-09-15 13:14:56]

reed@google.com changed reviewers:
+ danakj@chromium.org, mtklein@google.com

[reed1, 2014-09-15 13:37:46]

The CQ bit was checked by reed@google.com

[commit-bot: I haz the power, 2014-09-15 13:38:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/567393002/20001

[commit-bot: I haz the power, 2014-09-15 13:38:09]

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 2014-09-15 19:38 UTC

[mtklein, 2014-09-15 13:41:45]

lgtm

[commit-bot: I haz the power, 2014-09-15 13:44:50]

Committed patchset #2 (id:20001) as dee6a8e67db39fcbde2b3bb09be1d088ebb9db8a

[reed1, 2014-09-15 17:14:41]

A revert of this CL (patchset #2 id:20001) has been created in
https://codereview.chromium.org/569303002/ by reed@google.com.

The reason for reverting is: crashes on android bots, haven't diagnosed yet.