Avoid constructing scrollbars just to measure their thickness.

Introduce the method HyoptheticalScrollbarThickness, which avoids constructing a scrollbar. This
might be useful in figuring out overflow:auto scrollbars as well (left for a future CL).

R=skobes@chromium.org
BUG=823365

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Id7d3a1be44f7146bfbb2d650e9fa0d647db54f3f
Reviewed-on: https://chromium-review.googlesource.com/1110892
Commit-Queue: Stefan Zager <szager@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569705}
7 files changed