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This is a reland of 5fc3b9d0
Original change's description:
> [DevTools] Implement waitForDebugger for OOPIFs
>
> 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/818076
> Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
> Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#523835}

TBR=pfeldman

Bug: 750901
Change-Id: I96e6c083aa0de96799fbd4a529ae97019738ed87
Reviewed-on: https://chromium-review.googlesource.com/826318Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524191}