Add class WebScrollbarThemeClient

This is an abstraction to facilitate moving the bulk of platform/scroll
into core. WebScrollbarTheme provides a Mac-only API which is called
into from content, and affects the styling of all scrollbars. After
this patch, all of the other scrollbar and scrollbar theme code can move
out of platform.

BUG=823365
R=skobes@chromium.org,chrishtr@chromium.org
TBR=dgozman@chromium.org

Change-Id: I888e08676b5343156ae391c44b2084e90023de0f
Reviewed-on: https://chromium-review.googlesource.com/1141116
Commit-Queue: Stefan Zager <szager@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577102}
7 files changed