Get rid of GraphicsLayer::scrollable_area_

The only place where it was actually used was in
GraphicsLayer::SetScrollbarsHiddenIfOverlay, so just forward that
call to the GraphicsLayerClient.

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

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I3cdd24ceb1ab51563dbc3b6d9ce50ce9edc77bf8
Reviewed-on: https://chromium-review.googlesource.com/1132536
Commit-Queue: Stefan Zager <szager@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574587}
12 files changed