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Some extension/app pages are hosted by guest views, which means they
have nested web contents. This CL checks nested web contents and always
use the outer contents to handle print requests so the entire content
can be printed correctly.

BUG=819583, 447941

Change-Id: I6b45f4867d13cf5141476a7e748e8c0e5d3b0996
Reviewed-on: https://chromium-review.googlesource.com/969949
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545077}