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When an out-of-process subframe itself also has out-of-process
subframe(s), we need to be able to embed such contents too. This
CL adds a test to make sure our code works in such cases.

BUG=853942

Change-Id: I09d1fab4f10e299d3a7fb7f164bbbcfade3eebb5
Reviewed-on: https://chromium-review.googlesource.com/1166032
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582027}