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We should flush WebContents state between PendingAppManagerImpl tasks.
This fix is inspired by crbug.com/1086778, crrev.com/c/2237542 and
crrev.com/c/2246044.

Before each Install task and registration task, navigate WebContents
to about:blank to reset it. This hopefully fix InstallURLRedirected
errors spike on System Web Apps since BMO launch.

Bug: 1093707
Change-Id: Ia72f39ecc099c486014342c01ece5cbb4f0362f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2251418
Commit-Queue: Jiewei Qian  <qjw@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781201}