Fix memory issues in compositor_unittests

Fix memory issues in compositor_unittests

These LayerDelegates are trying to paint rectangles at offsets, but end
up always painting their rectangles at the layer origin.  This causes
any scheduled paint not at the origin to not fill the entire
invalidation, leading to uninitialized pixels being drawn.

The fix is rather than drawing a rect at the origin, just fill the
entire clip with the right color.  The clip is in the right place, so
this will do what the test intends.

Also fill the canvas in the DrawTreeLayerDelegate, which was always
leaving it uninitialized.

R=piman@chromium.org,danakj@chromium.org
BUG=421107
Committed: https://crrev.com/9b779de87d09341bb31eca53c8097a0f37f97c2d
Cr-Commit-Position: refs/heads/master@{#298817}

[piman, 2014-10-08 22:20:29]

LGTM, thanks!

[danakj, 2014-10-08 22:20:42]

LGTM 2

[enne (OOO), 2014-10-08 22:27:21]

The CQ bit was checked by enne@chromium.org

[commit-bot: I haz the power, 2014-10-08 22:29:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/637933004/1

[commit-bot: I haz the power, 2014-10-09 02:54:35]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-10-09 02:54:36]

Exceeded time limit waiting for builds to trigger.

[enne (OOO), 2014-10-09 06:15:54]

The CQ bit was checked by enne@chromium.org

[commit-bot: I haz the power, 2014-10-09 06:17:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/637933004/1

[commit-bot: I haz the power, 2014-10-09 06:26:44]

Committed patchset #1 (id:1) as 33d3a1d49becb002ca38bf73649e2c7dee767f76

[commit-bot: I haz the power, 2014-10-09 06:27:59]

Patchset 1 (id:??) landed as
https://crrev.com/9b779de87d09341bb31eca53c8097a0f37f97c2d
Cr-Commit-Position: refs/heads/master@{#298817}