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This CL adds an HTMLPortalElement to represent portals, and creates a mojo
connection to the browser-exposed interface when the element is attached
to the document.

Bug: 865565
Change-Id: Ib4e8053969d3e7ec138db84aae211b9a97a19b3c
Reviewed-on: https://chromium-review.googlesource.com/1167111
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583752}