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This is a hack to workaround a blink bug where overlay scrollbars aren't
rendered correctly in print preview. This bug is being looked at but
until then we use custom scrollbars that imitate mac scrollbars only
in this specific case.

BUG=466039

Review URL: https://codereview.chromium.org/1036943004

Cr-Commit-Position: refs/heads/master@{#322331}