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Before this CL, we were creating a SupervisedUserResourceThrottle for
each top-level navigation. If the current user isn't supervised, this is
pointless. One side-effect was that we were recording
ManagerUsers.Filter* histograms for all navigations, including for
non-supervised users.

This CL adds an "enabled" field on SupervisedUserURLFilter, which is set
to true iff the user is supervised. This allows us to not create the
resource throttle for regular users.

BUG=671472

Review-Url: https://codereview.chromium.org/2564043002
Cr-Commit-Position: refs/heads/master@{#437868}