Merge "arc: Implement scrollable OptIn window content."

Merge "arc: Implement scrollable OptIn window content."

> Support low resolutions by making OptIn content scrollable in case
> Chrome desktop resolution is lower than required for normal OptIn
> display. This also implements automatic window bounds update on
> desktop resoluton change.
>
> BUG=b/29817065
> TEST=Manually on devices.
> CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation
>
> Review-Url: https://codereview.chromium.org/2142933006
> Cr-Commit-Position: refs/heads/master@{#404947}
> (cherry picked from commit f8e016bfad2544663aab1bdcccd652477cbfbfe2)

BUG=633605