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This new class manages connecting the policy system with BrowserState
(user-level) prefs. ChromeBrowserStateImpl directly owns its
BrowserStatePolicyConnector and creates one before bringing up prefs. (A
KeyedService cannot be used because BrowserStatePolicyConnector needs to
outlive prefs.)

The overall set of classes and factories was chosen to mimic
desktop. They provide the necessary structure for implementing cloud
policy as a potential feature in the future.

BUG=1024115

Change-Id: I6addab6e877e406006702a581414bf20b7c6cfaa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067871Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744696}