Remove incorrect locking when calling WebAudioCapturerSource::removeFromBlinkSource()
Remove incorrect locking when calling WebAudioCapturerSource::removeFromBlinkSource()

Blink sometimes invokes WebAudioCapturerSource methods while holding
a Blink-side lock. This results in the WebAudioCapturerSource lock being acquired
after the Blink lock.

removeFromBlinkSource() calls Blink, acquiring a Blink-side lock.
It is important that the WebAudioCapturerSource lock is not held at that
time, as it would result in the same locks as the previous case acquired
in a different order, which may result in a deadlock.

BUG=506775

Review URL: https://codereview.chromium.org/1240233002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#339618}

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