Enforce thread-safety of bitmaps in pictures via the type.

Enforce thread-safety of bitmaps in pictures via the type.

No runtime difference here, but it makes it impossible to forget to make
a shallow copy; you can't get at the full bitmap without it.

NOTREECHECKS=true

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/f55c314bfa6db980941f8d0ab485beb2e08589ab

[mtklein, 2014-12-11 20:05:31]

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

[mtklein, 2014-12-11 20:05:45]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-12-11 20:05:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/799603002/1

[commit-bot: I haz the power, 2014-12-11 20:05:52]

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 2014-12-12 02:05 UTC

[reed1, 2014-12-11 20:33:09]

lgtm

[mtklein, 2014-12-11 20:42:45]

The CQ bit was unchecked by mtklein@google.com

[mtklein, 2014-12-11 20:42:47]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-12-11 20:42:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/799603002/1

[commit-bot: I haz the power, 2014-12-11 20:43:10]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/f55c314bfa6db980941f8d0ab485beb2e08589ab