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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}