Remove unused gfx::Canvas API

Remove unused gfx::Canvas API

Canvas::BeginPlatformPaint() and EndPlatformPaint() have no callers.

R=danakj@chromium.org
CC=djsollen@chromium.org

Committed: https://crrev.com/35f97018b601b2c17d7408397365c67c8c5099e7
Cr-Commit-Position: refs/heads/master@{#353889}

[Tom Hudson, 2015-10-13 19:11:42]

tomhudson@chromium.org changed reviewers:
+ tomhudson@chromium.org

[Tom Hudson, 2015-10-13 19:11:43]

Dana, PTAL - we're trying to clean up skia/ext/, and removing dead code calling
into it can only help.

[danakj, 2015-10-13 21:00:08]

LGTM

[tomhudson, 2015-10-13 21:36:44]

The CQ bit was checked by tomhudson@google.com

[commit-bot: I haz the power, 2015-10-13 21:44:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1403973002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1403973002/1

[commit-bot: I haz the power, 2015-10-13 22:58:02]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-13 22:58:40]

Patchset 1 (id:??) landed as
https://crrev.com/35f97018b601b2c17d7408397365c67c8c5099e7
Cr-Commit-Position: refs/heads/master@{#353889}