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This CL adds a policy handler for the HomepageLocation policy that
filters out invalid URLs and URLs with a non-standard scheme. It used
to be possible to set Javascript as homepage, which would then execute
in the context of the current page.

This filter applies to the policy only. The user can still use
Javascript in the homepage settings if they want to.

BUG=b:112186030
TEST=Set Javascript homepage through policy, made sure it's rejected in
      chrome://policy and policy is unset in settings; unit tests.

Change-Id: I991b2de55b7edaf67413b7695da7c566e8369600
Reviewed-on: https://chromium-review.googlesource.com/c/1186643
Commit-Queue: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613049}