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When a worker is stuck in JS when we call AttachDevToolsSession, the
the call won't go through until JS execution finishes as there is no
interrupting mechanism (equivalent to io_session) at the Agent level.

This CL adds different behaviour to DevToolsAgent for the worker and
non-worker cases. Non-workers now use the IO thread to dispatch calls
from the browser over the DevToolsAgent mojo interface. This is similar
to how IOSession works and achieves the same results.

We also send all protocol messages on IOSession for workers. We can't do
this for non-workers as they need to have an ordering guarantee with
page navigation. This is done through using associated mojo channels for
Session, Agent and FrameNavigationControl. We don't associate the mojo
channel for workers though.

Bug: 1010534
Change-Id: Id6b3d2ee0cf86f1a0b046b4e59ec1851ca07d7eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1951003Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733963}