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This reduces some setup and layer plumbing because the WebSandboxSupport
is now implemented on top of Mojo.

Change-Id: Ib969ce301963151d6abb048aaa0319283b987b57
Reviewed-on: https://chromium-review.googlesource.com/c/1351727Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611766}