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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.

BUG=chromium:839242
TEST=unit test included in this CL

Change-Id: I1428a49cce918930f8b5d4083fbc9ba208e8b101
Reviewed-on: https://chromium-review.googlesource.com/c/1393547
Commit-Queue: Tim Zheng <timzheng@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619728}