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This CL updates the documentation for WebUI to reflect recent changes
that make it safe to use <iframe> to embed web content in a WebUI page.

Bug: 1002276
Change-Id: I1d61d99dd6f71b868f3ae05d3e37c801c6095d45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613888
Auto-Submit: Nasko Oskov <nasko@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841421}