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

browser/base/content/tabbrowser.js

@@ -4431,6 +4431,16 @@ window._gBrowser = {
         tab.finishMediaBlockTimer();
       }
     });
+
+    this.addEventListener("AudibleAutoplayMediaOccurred", (event) => {
+      let browser = event.originalTarget;
+      let tab = this.getTabForBrowser(browser);
+      if (!tab) {
+        return;
+      }
+
+      Services.obs.notifyObservers(tab, "AudibleAutoplayMediaOccurred");
+    });
   },
 };