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This CL adds a flag which will allow the website to request the
'elevated' It2Me host (with the UiAccess manifest value) is used
instead of the base It2Me host which does not have access to
elevated windows.  The flag is overridden by the Chrome policy so
if a network admin has specified that they do not want to allow
elevated remote support hosts, this new flag is ignored.

Change-Id: Ic84968f432db6ab041a5b4afdc38a2ac7be27a50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586967
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837138}