Web Audio: when media playback requires a user gesture, apply rule to cross origin iframes.

third_party/WebKit/Source/modules/webaudio/AudioBufferSourceNode.cpp

 /*
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  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
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  * 1.  Redistributions of source code must retain the above copyright
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@@ skipping 430 matching lines @@
 
 void AudioBufferSourceHandler::start(double when, double grainOffset, double grainDuration, ExceptionState& exceptionState)
 {
     startSource(when, grainOffset, grainDuration, true, exceptionState);
 }
 
 void AudioBufferSourceHandler::startSource(double when, double grainOffset, double grainDuration, bool isDurationGiven, ExceptionState& exceptionState)
 {
     DCHECK(isMainThread());
 
-    context()->recordUserGestureState();
+    context()->startRenderingIfNeeded();

[Raymond Toy, 2016/09/07 16:13:19]

I don't think this is right.  When the context is created, startRendering() is
called immediately when the destination is created.  And if I understand what
you're trying to do, this might also interfere with any OfflineAudioContexts. 
These contexts shouldn't be require a gesture because they don't output any
audio.

[mlamouri (slow - plz ping), 2016/09/08 13:12:42]

When the AudioContext is created, it might not be rendering yet if there was no
user gesture. In this case, start() should resume automatically. 

Regarding offline audio context, I answered below: it's for compat with Safari
iOS. If both of you think there is really no reason to do this, let me know and
I can change that.

 
     if (playbackState() != UNSCHEDULED_STATE) {
         exceptionState.throwDOMException(
             InvalidStateError,
             "cannot call start more than once.");
         return;
     }
 
     if (when < 0) {
         exceptionState.throwDOMException(
@@ skipping 228 matching lines @@
 {
     audioBufferSourceHandler().start(when, grainOffset, exceptionState);
 }
 
 void AudioBufferSourceNode::start(double when, double grainOffset, double grainDuration, ExceptionState& exceptionState)
 {
     audioBufferSourceHandler().start(when, grainOffset, grainDuration, exceptionState);
 }
 
 } // namespace blink