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Gives hosted webviews the same facility that normal cast applications
have to enable devtools debugging for themselves and their inner
contents.

Bug: internal b/152230325
Test: manual
Change-Id: I80b60c0d26d5de6183325c7dfb36ced3b459de35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2136140
Commit-Queue: Ryan Daum <rdaum@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Sean Topping <seantopping@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756823}