Mark SkPictureRecorder::endRecording as SK_WARN_UNUSED_RESULT.

Mark SkPictureRecorder::endRecording as SK_WARN_UNUSED_RESULT.

Ignoring the result is a memory leak.
Callers should always at least unref the result.

This bug has occurred in Blink:
  https://codereview.chromium.org/1068413002/

BUG=skia:3680
Committed: https://skia.googlesource.com/skia/+/7d1b945da6afdfb50372fd7b69153710d7793a52

[jbroman, 2015-04-08 21:08:32]

The CQ bit was checked by jbroman@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-04-08 21:08:56]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1072633002/1

[commit-bot: I haz the power, 2015-04-08 21:08:57]

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 2015-04-09 03:08 UTC

[mtklein, 2015-04-08 21:09:43]

mtklein@google.com changed reviewers:
+ mtklein@google.com

[mtklein, 2015-04-08 21:09:44]

lgtm

[mtklein, 2015-04-08 21:12:21]

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

[mtklein, 2015-04-08 21:12:21]

+reed, who the CQ will want to LGTM this too.

[commit-bot: I haz the power, 2015-04-08 21:28:11]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-04-08 21:28:12]

This issue passed the CQ dry run.

[reed1, 2015-04-09 13:11:28]

lgtm

[reed1, 2015-04-09 13:11:52]

we should consider this for all of our factory-ish methods (and factories)

[reed1, 2015-04-09 13:12:01]

The CQ bit was checked by reed@google.com

[commit-bot: I haz the power, 2015-04-09 13:12:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1072633002/1

[commit-bot: I haz the power, 2015-04-09 13:12:23]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/7d1b945da6afdfb50372fd7b69153710d7793a52