ui: Remove the canvas() member from PaintContext.

ui: Remove the canvas() member from PaintContext.

The canvas() is now only accessed through the Recorder classes (ie.
PaintRecorder and friends).  Now we can put a DisplayItemList instead
of a Canvas inside PaintContext.

R=piman@chromium.org
BUG=466426
Committed: https://crrev.com/a002bde549dd2edf1a5c1487c3711730937edf52
Cr-Commit-Position: refs/heads/master@{#324210}

[piman, 2015-04-07 23:19:32]

lgtm

[danakj, 2015-04-08 08:06:27]

The CQ bit was checked by danakj@chromium.org

[commit-bot: I haz the power, 2015-04-08 08:06:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1069183002/1

[commit-bot: I haz the power, 2015-04-08 10:53:46]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-08 10:54:36]

Patchset 1 (id:??) landed as
https://crrev.com/a002bde549dd2edf1a5c1487c3711730937edf52
Cr-Commit-Position: refs/heads/master@{#324210}