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Makes the basic policy infrastructure changes to begin supporting the
BrowserSignin policy. The policy handler is hidden behind a command-line
switch, since the policy isn't fully supported yet. Attempting to sign
in while setting this policy to 0 (sign-in disabled) will make the
browser crash, as not all parts of the authentication flow correctly
handles sign-in being disabled yet.

Change-Id: Iefad6d36685bb11a26e55494dc3be4222e8366c5
Bug: 1155745
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2562719
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835694}