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This CL introduces a new base::Feature to control whether the sign-in
origin (https://accounts.google.com) requires a dedicated process, to
be used for a Finch experiment.  As part of this, during
initialization content/ will call out to a new ContentBrowserClient
method to get a whitelist of origins requiring a dedicated process.
These origins will be process-isolated in all profiles for the
lifetime of the browser.

Bug: 713444, 739418
Change-Id: I308cd1eb209394a9990c5eb5793a95a78d76485a
Reviewed-on: https://chromium-review.googlesource.com/569519
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Nick Carter <nick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487263}