Fix threading races on WebAudioSourceProviderImpl::provideInput

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.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=157259

[haraken, 2013-09-04 23:15:01]

This is already reviewed in https://chromiumcodereview.appspot.com/23691033/

dale, kbr: would you rubberstamp?

[Ken Russell (switch to Gerrit), 2013-09-04 23:23:16]

lgtm

[commit-bot: I haz the power, 2013-09-04 23:26:21]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/haraken@chromium.org/23969007/1

[DaleCurtis, 2013-09-05 00:14:47]

lgtm

[commit-bot: I haz the power, 2013-09-05 02:07:01]

Change committed as 157259