cc: Return a NullCanvas instead of NULL in context lost situations.

cc: Return a NullCanvas instead of NULL in context lost situations.

This reduces the difference in code paths between normal usage and
context lost situations.

Also moves the save()/restore() pair required for GPU raster
into the RasterBuffer class so it can be avoided when unnecessary.

BUG=406404
Committed: https://crrev.com/102706b5cedfa35c68069f185a4f8c187d485722
Cr-Commit-Position: refs/heads/master@{#295290}

[reveman, 2014-09-17 14:18:02]

reveman@chromium.org changed reviewers:
+ vmpstr@chromium.org

[vmpstr, 2014-09-17 15:26:42]

lgtm

[reveman, 2014-09-17 15:29:24]

The CQ bit was checked by reveman@chromium.org

[commit-bot: I haz the power, 2014-09-17 15:30:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/578843002/20001

[commit-bot: I haz the power, 2014-09-17 17:20:05]

Committed patchset #2 (id:20001) as 1daf9f1d4b376d9a8cd4b9534560bb3399829239

[commit-bot: I haz the power, 2014-09-17 17:20:42]

Patchset 2 (id:??) landed as
https://crrev.com/102706b5cedfa35c68069f185a4f8c187d485722
Cr-Commit-Position: refs/heads/master@{#295290}