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Refactoring of ProtectedMediaIdentifierPermissionContext to derive from
PermissionContextBase class to conform with other APIs like geolocation
and midi. This has the advantage of less code and a consistent path for
all permissions. Also it makes it easier to implement PermissionService
because some functionality like HasPermission() can be implemented at
the level of PermissionContextBase.

Also add the corresponding UMA bits in histograms.xml and
permission_context_uma_util.cc.

BUG=

Review URL: https://codereview.chromium.org/769103002

Cr-Commit-Position: refs/heads/master@{#308100}