[New Multicolumn] Clip correctly if the "root" layer is inside the multicol container.

[New Multicolumn] Clip correctly if the "root" layer is inside the multicol container.

When the "root" layer is inside the multicol container, don't get lost and
make the clip rectangle relative to the RenderView. It needs to be relative
to the "root" layer. When compositing, the "root" layer could be the very
same as the layer being painted.

BUG=359877
R=chrishtr@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=188552

[chrishtr, 2015-01-16 18:40:40]

The CQ bit was checked by chrishtr@chromium.org

[chrishtr, 2015-01-16 18:40:40]

lgtm

[commit-bot: I haz the power, 2015-01-16 18:41:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/848163002/1

[commit-bot: I haz the power, 2015-01-16 22:08:08]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=188552