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This adds tests to PermissionDialogTests that test Media requests with
the kUsePermissionManagerForMediaRequests feature enabled. When this
feature is enabled, media permissions can be requested in the same way
as other permissions.

Once the feature flag is removed, the old behavior that is being tested
can also be removed.

BUG=596786

Review-Url: https://codereview.chromium.org/2838873004
Cr-Commit-Position: refs/heads/master@{#468283}