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This CL makes sure that SupervisedUserNavigationObserver is
created only for supervised user accounts. This saves memory
and processing time.

Bug: 1043232
Change-Id: I5d13ce0261d07c29384f5ba5dc83e420ed3138dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2008315Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Dan S <danan@chromium.org>
Commit-Queue: Yilkal Abe <yilkal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733750}