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The current PolicyWatcher exposes a set of 'current' policies which
is actually the default policies with overrides from any platform
policy which is actually set.  This works fine for driving behavior
but in some cases we actually want to know whether a policy value
we receive is the 'default' or if an admin has set it explicitly.

To allow this, the PolicyWatcher will now store the set of platform
policies on the machine and provide it on request.  I plan to use
this for logging (to get an idea of policy usage and popularity)
and for cases where we want to provide a flag from the website to
the NMH but also want admins to override the value if needed.

Since the PolicyWatcher provides the platform policies now, I renamed
GetCurrentPolicies() to GetEffectivePolicies() as I think that name
reflects the fact that it is a merged set of policies.

Change-Id: Id2fd578ac42d925922e8beb5f028374d7bcdcbf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587614Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837116}