Fix threading races on WebAudioSourceProviderImpl::provideInput

Fix threading races on WebAudioSourceProviderImpl::provideInput 

According to the crash report (https://cluster-fuzz.appspot.com/testcase?key=4697390229487616), 
there is a threading race. Specifically, WebAudioSourceProviderImpl can be destructed by the main thread while WebAudioSourceProviderImpl::Stop() is being called by the audio thread. 

The core problem is that we're not calling WebAudioSourceProviderImpl::setClient(NULL) when HTMLMediaElement clears the audio source provider. 

BUG=284786
No tests because the crash depends on threading races and thus not reproducible.

Review URL: https://chromiumcodereview.appspot.com/23969007

git-svn-id: svn://svn.chromium.org/blink/trunk@157259 bbb929c8-8fbe-4397-9dbb-9b2b20218538