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This allows us to reuse dispatching on a custom channel talking
to some DevTools backend.

This needs follow up to remove ExternalHost and unnecessary methods
in HeadlessDevToolsTarget once clients migrate away from it.

Change-Id: Ia32d8feef4dc3f9d85db75b556242f0fbd9428ff
Reviewed-on: https://chromium-review.googlesource.com/1097951Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567453}