Oilpan: address ScrollableArea-induced build breakage from r194957.

Oilpan: address ScrollableArea-induced build breakage from r194957.

ScrollableArea is not an Oilpan allocated mixin currently, so in order
to support the assumption of RootFrameViewport that all ScrollableAreas
are allocated the same way, pull PinchViewport off the Oilpan heap
(PinchViewport being the only ScrollableArea that was Oilpan controlled.)

This is not ideal, but a shorter term measure to unbreak Oilpan builds.
Making ScrollableArea a GC mixin would be the longer term solution, but
that's not a straightforward task.

TBR=oilpan-reviews
BUG=443724
NOTRY=true

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=194964

[sof, 2015-05-06 06:11:52]

please take a look.

[sof, 2015-05-06 06:12:00]

The CQ bit was checked by sigbjornf@opera.com

[commit-bot: I haz the power, 2015-05-06 06:12:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1127023002/1

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

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=194964

[haraken, 2015-05-06 13:57:39]

LGTM as a short-term fix. Thanks!