Set |Channel::is_shutting_down_| during channel shutdown.
Set |Channel::is_shutting_down_| during channel shutdown.

This flag is set by calling |Channel::WillShutdownSoon()|, but that may
not be called in all cases (i.e. on broken connection due to renderer
crash). crrev.com/95049185be27f0da5e9d2921a1be2b91f72b7956 relies on
this flag being set on shutdown to prevent a race with
|Channel::AttachAndRunEdpoint()|.

Review URL: https://codereview.chromium.org/1367383002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#351013}

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