Add autotest API for setting a Crostini app scaled.

To set a Crostini app scaled is to set the property in the Crostini app
registry so that when the app launches it'll use low display density.

TEST=unit test included in this CL

Change-Id: I1428a49cce918930f8b5d4083fbc9ba208e8b101
Commit-Queue: Tim Zheng <>
Reviewed-by: Ben Wells <>
Reviewed-by: Dan Erat <>
Cr-Commit-Position: refs/heads/master@{#619728}
6 files changed