Since we already save the overlay scrollbar color theme in ScrollableArea for
Mac, this patch plumbs it through for Aura overlay scrollbars.

Also we rename ScrollbarOverlayStyle to ScrollbarOverlayColorTheme to avoid
misleading since we already have ScrollbarStyle WebThemeEngine.h.

BUG=636662
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2426793002
Cr-Commit-Position: refs/heads/master@{#428367}