cc: Make unittests use FakePictureLayer instead of FakeContentLayer.

cc: Make unittests use FakePictureLayer instead of FakeContentLayer.

The latter is going away. Long live Implside painting. This
fixes cc/trees/layer_tree_host_unittest_animation.cc,
cc/trees/layer_tree_host_unittest_copyrequest.cc and
cc/trees/layer_tree_host_common_unittest.cc.

R=enne, vmpstr
BUG=413479
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Committed: https://crrev.com/4e95f763c7f52664c94373b7d9fcc0ed96c7f019
Cr-Commit-Position: refs/heads/master@{#333111}

[enne (OOO), 2015-06-05 18:14:00]

lgtm

[vmpstr, 2015-06-05 18:16:18]

lgtm 2

[danakj, 2015-06-05 18:16:34]

The CQ bit was checked by danakj@chromium.org

[commit-bot: I haz the power, 2015-06-05 18:18:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1169643002/20001

[commit-bot: I haz the power, 2015-06-05 19:46:33]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-05 20:00:23]

Patchset 2 (id:??) landed as
https://crrev.com/4e95f763c7f52664c94373b7d9fcc0ed96c7f019
Cr-Commit-Position: refs/heads/master@{#333111}