WebAudio: record whether user gesture was available and provided.

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

 /*
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  *
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@@ skipping 423 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)
 {
     ASSERT(isMainThread());
 
+    context()->recordUserGesture();

[Raymond Toy, 2016/06/03 17:57:52]

Not sure if this is the right place.  Say you've created a context and set up a
bunch of sources.  Then you schedule the source to start 1 minute from now. 
You'll get the user gesture recorded, but the sound wouldn't have started
playing for another minute.  

Don't you want to know that when the sound actually starts playing that you
didn't have the user gesture permission to play?

[mlamouri (slow - plz ping), 2016/06/05 14:44:57]

As above, I'm following WebKit's behaviour here. The rationale is that if
start() is called with a gesture, it will unlock the AudioContext which will
then be able to start running when startPlaying() is called. Otherwise, if
`start(1000)` is called with a gesture, when the playback actually starts, there
will be no gesture on the stack. Does that make sense?

[Raymond Toy, 2016/06/06 15:10:07]

Acknowledged.

+
     if (playbackState() != UNSCHEDULED_STATE) {
         exceptionState.throwDOMException(
             InvalidStateError,
             "cannot call start more than once.");
         return;
     }
 
     if (when < 0) {
         exceptionState.throwDOMException(
             InvalidStateError,
@@ skipping 227 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