Record root layer's position in its TransformNode

Record root layer's position in its TransformNode

Currently we only record non-root layer's position in the
associated TransformNode. In practice, root layer might always
be positioned at (0,0), so we might not be seeing the problem.
But in tests we can manually set root layer to have non-zero
position which will cause visible rects mismatch for property
tree verification.

One example is the unittest in this CL that root layer is positioned
at (60, 70, 100x100). Viewport size is (0, 0, 100x100) in target
space, so the visible rects of the root layer is clipped to be
size 40x30.

With the fix the unittest pass with property tree verification
turned on.

BUG=None
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Committed: https://crrev.com/9d3da5243ea13e8c057dac2b1a4d61285182eb11
Cr-Commit-Position: refs/heads/master@{#333109}

[Yufeng Shen (Slow to review), 2015-06-04 22:36:03]

miletus@chromium.org changed reviewers:
+ vollick@chromium.org

[enne (OOO), 2015-06-05 17:43:43]

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

[enne (OOO), 2015-06-05 17:43:44]

The CQ bit was checked by enne@chromium.org

[enne (OOO), 2015-06-05 17:43:44]

lgtm

[commit-bot: I haz the power, 2015-06-05 17:45:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1159983010/20001

[commit-bot: I haz the power, 2015-06-05 19:45:19]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-05 20:00:16]

Patchset 2 (id:??) landed as
https://crrev.com/9d3da5243ea13e8c057dac2b1a4d61285182eb11
Cr-Commit-Position: refs/heads/master@{#333109}