Fold PaintInvalidationCapableScrollableArea into PLSA.

Bug: 823365
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ic7d1091222850e65d5442b7a339c6925d5c51dac
Reviewed-on: https://chromium-review.googlesource.com/1110882
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569476}
10 files changed