Make SkEmptyPicture a singleton.

Make SkEmptyPicture a singleton.

In my confusion yesterday I accidentally left this as a non-singleton.
The issue in Blink was not related to this being a singleton,
and it should be safe to make it one.

This means recording an empty picture properly costs zero mallocs.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/2e172ecd3b0ed570f6bf27594c524ea63a606066

[mtklein_C, 2015-05-20 16:45:53]

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

[commit-bot: I haz the power, 2015-05-20 16:46:06]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1147053002/1

[mtklein_C, 2015-05-20 16:46:36]

mtklein@chromium.org changed reviewers:
+ fmalita@chromium.org

[commit-bot: I haz the power, 2015-05-20 16:51:48]

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

[commit-bot: I haz the power, 2015-05-20 16:51:49]

Dry run: This issue passed the CQ dry run.

[f(malita), 2015-05-20 17:15:41]

lgtm

[mtklein, 2015-05-20 17:26:01]

The CQ bit was checked by mtklein@google.com

[mtklein, 2015-05-20 17:26:02]

The patchset sent to the CQ was uploaded after l-g-t-m from fmalita@chromium.org
Link to the patchset: https://codereview.chromium.org/1147053002/#ps20001
(title: "rebase")

[commit-bot: I haz the power, 2015-05-20 17:26:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1147053002/20001

[commit-bot: I haz the power, 2015-05-20 17:32:26]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/2e172ecd3b0ed570f6bf27594c524ea63a606066