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|controller_binding_| may be unbound during WebComponent teardown,
causing a FIDL write error to be logged, if:
- The component was started without a ComponentController request.
- The ComponentController was previously Detach()ed and closed.
- The WebComponent is tearing down because the ComponentController
  channel was closed by the client.

The OnTerminated event is now dispatched only if |controller_binding_|
is still bound.

Bug: 1157638
Change-Id: Idd15e6cb07a98a27a56d3b5a9f8c1c611b04c816
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2584849
Auto-Submit: Wez <wez@chromium.org>
Commit-Queue: David Dorwin <ddorwin@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836171}