Hide pinch viewport scrollbars when near minimum scale.

Hide pinch viewport scrollbars when near minimum scale.

This patch prevents the overlay scrollbars from showing up when we're near the
minimum scale. The threshold can be set per-platform; by default it's 0 so the
scrollbars will be visible regardless of the scale. On Windows/Linux/CrOS we'd
like to prevent the scrollbars from overlaying over the window scrollbars so
we set a small non-0 threshold.

BUG=446411
Review URL: https://codereview.chromium.org/877863002

Cr-Commit-Position: refs/heads/master@{#313431}
(cherry picked from commit c784a6f02ed8f2cffe131c2c067c4805cd752610)

Committed: https://chromium.googlesource.com/chromium/src/+/5198bfa1b299cce3eb2cccab2714f2f5465dd4b6

[bokan, 2015-01-29 15:19:04]

Committed patchset #1 (id:1) manually as
5198bfa1b299cce3eb2cccab2714f2f5465dd4b6.