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Using NavigationThrottle for this. Since during throttling
the navigation has not committed yet, we have to create
agent host for soon-to-be-committed frame in advance.

Bug: 750901
Change-Id: I88dbd982481b88eeec36209b6bf3ad7f74863200
Reviewed-on: https://chromium-review.googlesource.com/818076Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523835}