Bug 1476701 - notify observer when audible autoplay occurred. r=cpearce,jaws

In our autoplay shield-study, we want to collect the information which could tell us how many website
contains audible autoplay media, but there is no way to get this information on current API desigin.

Therefore, I would like to send a new notification when autoplay occurred.

The extension code could get the information by following way,
```
Services.obs.addObserver((subject, topic, data) => {
    // DO SOMETHING
}, "AudibleAutoplayMediaOccurred");
```

MozReview-Commit-ID: 4bSYcxDZOGK

dom/html/HTMLMediaElement.cpp

@@ -4013,7 +4013,7 @@ HTMLMediaElement::AudioChannelAgentDelayingPlayback()
 }
 
 void
-HTMLMediaElement::ReportAutoplayTelemetry() const
+HTMLMediaElement::UpdateHadAudibleAutoplayState() const
 {
   // If we're audible, and autoplaying...
   if ((Volume() > 0.0 && !Muted()) &&
@@ -4086,7 +4086,7 @@ HTMLMediaElement::Play(ErrorResult& aRv)
     return promise.forget();
   }
 
-  ReportAutoplayTelemetry();
+  UpdateHadAudibleAutoplayState();
 
   const bool handlingUserInput = EventStateManager::IsHandlingUserInput();
   switch (AutoplayPolicy::IsAllowedToPlay(*this)) {
@@ -6246,7 +6246,7 @@ HTMLMediaElement::CheckAutoplayDataReady()
     return;
   }
 
-  ReportAutoplayTelemetry();
+  UpdateHadAudibleAutoplayState();
   switch (AutoplayPolicy::IsAllowedToPlay(*this)) {
     case nsIAutoplay::BLOCKED:
       return;