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This change moves migrating of renamed policies from the PolicyHandler
to a PolicyMigrator that happens right after the PolicyMerger has run.
This guarantees that list policies can be properly merged between
sources even if different sources provide different naming.

BUG=1138572
TEST=browser_tests:PolicyPrefsTest.PolicyToPrefsMapping

Change-Id: Ibe335176ef52dabbe09c076ba57ae04d1c67c1c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2507771
Commit-Queue: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Owen Min <zmin@chromium.org>
Reviewed-by: Yann Dago <ydago@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826903}