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If another channel of Chromium is already default, don't ask to
become default.

This re-lands e2d335fb with a fix.

Bug: 1109417
Change-Id: Ib0c699ebbbc15e0bb636f6d49ea6082c3208f423
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2323824Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792347}