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Previously, ExtensionPolicyMigrators only ran on platform policies, and
machine-level user cloud policies. This leaves out user cloud policies,
and CrOS-specific policy providers.

This CL makes the list of ExtensionPolicyMigrators static, instead of
each provider keeping a separate list. This ensures no provider is left
out of the policy migrator logic.

TESTED=checked that the LBS extension picks up `BrowserSwitcherEnabled`
    when it's set through user cloud policy

Bug: 1043775, 1030607
Change-Id: Ib257d2f18e3a106cbcd04d957b84a09b005a4680
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2013485Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734461}