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This CL adds a basic browsertest to verify System Web Apps works after
a reinstall. This typically happens on a version upgrade.

This CL tests that the installation process completes without crashing,
and the installed Apps can be launched (with the exception of Terminal).

Terminal requires resources that are only available in Chrome OS images,
so we can't launch it from chromium source. Tast tests are needed for
this scenario.

Bug: 1095524
Change-Id: Ib7053ea56cf0eee8031c66b5d77edcc283236972
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316191Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: Jiewei Qian  <qjw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792221}