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This uses a PostMessageHandler internally to communicate with the web contents.
The PostMessageHandler is initialised once the dynamic module has been loaded
and is reset each time the web contents change.

A new class, ActivityDelegatePostMessageBackend, replaces the service backend
usually used by PostMessageHandler and forwards all events along to the
dynamic module if it's present.

Bug: 898840
Change-Id: Ia308c84cd771d2fdb96fc076472488fc48946162
Reviewed-on: https://chromium-review.googlesource.com/c/1323669
Commit-Queue: David Barker <barkerd@google.com>
Reviewed-by: Michael van Ouwerkerk <mvanouwerkerk@chromium.org>
Reviewed-by: Peter Conn <peconn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607587}