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This is a preparation for implementing a new LaunchSystemWebApp API,
where we use AppService to launch apps and don't return the Browser
object hosting the app (because AppService doesn't return it to us).

This change is safe for terminal, because LaunchSystemWebApp won't fail
if it is called with appropriate parameters after OnAppsSynchronized.

The metrics for CrostiniResult::UNKNOWN_ERROR contains other error
origins, and don't reflect whether LaunchSystemWebApp fails.

Bug: 1154540
Change-Id: I517202293140a7907f8af061ecbffec3dac623e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2576861Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Jiewei Qian  <qjw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836524}