SkImage_Raster's pixels are always immutable.

SkImage_Raster's pixels are always immutable.

To make this work, we tag their pixelrefs as temporarily immutable, allowing
ourselves to restore the pixels to mutability only when the image drops away.

This should allow us to wobble back and forth between writing to the Surface
and reading from the Image without a COW, with the Surface seeing mutable
pixels and the Image seeing immutable pixels.

The big idea is, Image doesn't need forever-immutable pixels, it just needs
pixels that are immutable as long as it's alive.

BUG=skia:

patch from issue 804523002 at patchset 40001 (http://crrev.com/804523002#ps40001)

Committed: https://skia.googlesource.com/skia/+/26e0e587f76f2a9338652c100f835c2377c908d3

[reed1, 2015-07-29 17:34:01]

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

[reed1, 2015-07-29 17:34:08]

The CQ bit was checked by reed@google.com

[commit-bot: I haz the power, 2015-07-29 17:34:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1254383006/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1254383006/20001

[commit-bot: I haz the power, 2015-07-29 17:34:27]

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 2015-07-29 23:34 UTC

[reed1, 2015-07-29 17:35:32]

The CQ bit was unchecked by reed@google.com

[mtklein, 2015-07-29 17:58:23]

lgtm % the segfault

[reed1, 2015-07-29 18:26:34]

The CQ bit was checked by reed@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-07-29 18:26:52]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1254383006/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1254383006/40001

[reed1, 2015-07-29 18:37:02]

The CQ bit was checked by reed@google.com

[reed1, 2015-07-29 18:37:02]

The patchset sent to the CQ was uploaded after l-g-t-m from mtklein@google.com
Link to the patchset: https://codereview.chromium.org/1254383006/#ps60001
(title: " ")

[commit-bot: I haz the power, 2015-07-29 18:37:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1254383006/60001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1254383006/60001

[commit-bot: I haz the power, 2015-07-29 18:44:55]

Committed patchset #4 (id:60001) as
https://skia.googlesource.com/skia/+/26e0e587f76f2a9338652c100f835c2377c908d3

[robertphillips, 2015-07-29 19:26:28]

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

[robertphillips, 2015-07-29 19:26:29]

lgtm

https://codereview.chromium.org/1254383006/diff/60001/src/core/SkPixelRef.cpp
File src/core/SkPixelRef.cpp (right):

https://codereview.chromium.org/1254383006/diff/60001/src/core/SkPixelRef.cpp...
src/core/SkPixelRef.cpp:367: fMutability = kMutable;
Hmmm .. can we get rid of the 'notifyPixelsChanged' ?