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This CL adds some basic tests that verify process model and security
expectations of WebUI. It adds basic bindings tests and a test for
iframing same WebUI page. More tests will be added to this as work
progresses on making WebUI be per frame and supporting site isolation.

Bug: 713313
Change-Id: Ib680cf85dbfdee26d40896588edd4d7364332e3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1835014Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703835}