fix: Fire PRFT event every time (#7408)

Always raise prft events so consumers can have access to the wall clock
time.

lib/media/media_source_engine.js

@@ -167,9 +167,6 @@ shaka.media.MediaSourceEngine = class {
 
     /** @private {!shaka.util.PublicPromise.<number>} */
     this.audioCompensation_ = new shaka.util.PublicPromise();
-
-    /** @private {boolean} */
-    this.parsedPrftEventRaised_ = false;
   }
 
   /**
@@ -496,7 +493,6 @@ shaka.media.MediaSourceEngine = class {
     this.firstAudioTimestamp_ = null;
     this.firstAudioReferenceStartTime_ = null;
     this.audioCompensation_ = new shaka.util.PublicPromise();
-    this.parsedPrftEventRaised_ = false;
 
     for (const contentType of streamsByType.keys()) {
       const stream = streamsByType.get(contentType);
@@ -837,11 +833,9 @@ shaka.media.MediaSourceEngine = class {
         const Mp4Parser = shaka.util.Mp4Parser;
         let startTime = 0;
         let parsedMedia = false;
-        const parser = new Mp4Parser();
-        if (!this.parsedPrftEventRaised_) {
-          parser.fullBox('prft', (box) => this.parsePrft_(timescale, box));
-        }
-        parser.box('moof', Mp4Parser.children)
+        new Mp4Parser()
+            .fullBox('prft', (box) => this.parsePrft_(timescale, box))
+            .box('moof', Mp4Parser.children)
             .box('traf', Mp4Parser.children)
             .fullBox('tfdt', (box) => {
               goog.asserts.assert(
@@ -905,7 +899,6 @@ shaka.media.MediaSourceEngine = class {
     const event = new shaka.util.FakeEvent(
         eventName, data);
     this.playerInterface_.onEvent(event);
-    this.parsedPrftEventRaised_ = true;
   }