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This CL alters the System App installation pipeline in order to allow
the full PWA setup experience to succeed for System Apps (mostly
adding exceptions for chrome:// URLs), allowing System Apps
to be installed with a scope. It also adds a browser test which
installs a System PWA end to end.

Bug: 836128
Change-Id: Ida32fa45484661d32a9607b44e06f28f272d4cc3
Reviewed-on: https://chromium-review.googlesource.com/c/1272636
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601837}