Invalidate the weak factory in SpeechRecognizer::EventListener::StopOnIOThread().
Invalidate the weak factory in SpeechRecognizer::EventListener::StopOnIOThread().

When an |app_list::SpeechRecognizer| is destroyed, a callback to
SpeechRecognizer::EventListener::StopOnIOThread() is posted to the IO
thread, and the |SpeechRecognizer| releases it's reference to
|EventListener|. |EventListener| may have weak pointers that must be
invalidated on the IO thread. However, on |SpeechRecognizer| releasing
its reference, the callback may have already run and hence the
|EventListener| is deleted upon releasing that reference on the UI
thread. This change ensures there are no more weak references to
|EventListener| by the time it is destroyed.

BUG=547650

Review URL: https://codereview.chromium.org/1413403004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#356008}

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