[RootLayerScrolls] Make scroll origin PLSA-specific.

PLSA is the only ScrollableArea subclass that ever gets a non-zero
scroll origin. Push all the logic down into PLSA.

BUG=823365
R=skobes@chromium.org,bokan@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I163b409a66c8b6d1dd248e51aae843cba109395e
Reviewed-on: https://chromium-review.googlesource.com/1114460
Commit-Queue: Stefan Zager <szager@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570949}
26 files changed