Merge 157259 "Fix threading races on WebAudioSourceProviderImpl:..."

Merge 157259 "Fix threading races on WebAudioSourceProviderImpl:..."

> 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

TBR=haraken@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=157694

[haraken, 2013-09-12 19:25:48]

Committed patchset #1 manually as r157694.