Adds a flag to open external links inside desktop PWAs

The flag `desktop-pwas-stay-in-window` controls whether
out of scope links open in an external browser. When
disabled (the default) behavior remains the same as
before this change (out of scope links open in the browser).
When enabled, out of scope links will open in the PWA
window and display the location bar (as we do already, in
bookmark apps).

This change has been made because the current flow (kicking
the user to the browser) breaks authentication flows on
some websites.

Bug: 853593
Change-Id: Id38dbd8437651133b2b5be5f05260b342066d0ce
Reviewed-on: https://chromium-review.googlesource.com/c/1242727
Commit-Queue: Jay Harris <harrisjay@chromium.org>
Reviewed-by: Matt Giuca <mgiuca@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597772}
8 files changed