Bug 1244639: P1. Don't assume MP3 decoding always starts at 0. r=cpearce

This remove the MP3 workaround for the mac decoder that was added in bug 1194080.

dom/media/MP3Demuxer.h

@@ -28,11 +28,6 @@ public:
   bool IsSeekable() const override;
   void NotifyDataArrived() override;
   void NotifyDataRemoved() override;
-  // Do not shift the calculated buffered range by the start time of the first
-  // decoded frame. The mac MP3 decoder will buffer some samples and the first
-  // frame returned has typically a start time that is non-zero, causing our
-  // buffered range to have a negative start time.
-  bool ShouldComputeStartTime() const override { return false; }
 
 private:
   // Synchronous initialization.

dom/media/MediaFormatReader.cpp

@@ -1571,15 +1571,12 @@ MediaFormatReader::GetBuffered()
   if (!mInitDone) {
     return intervals;
   }
-  int64_t startTime;
+  int64_t startTime = 0;
   if (!ForceZeroStartTime()) {
     if (!HaveStartTime()) {
       return intervals;
     }
     startTime = StartTime();
-  } else {
-    // MSE, start time is assumed to be 0 we can proceeed with what we have.
-    startTime = 0;
   }
   // Ensure we have up to date buffered time range.
   if (HasVideo()) {
@@ -1602,6 +1599,11 @@ MediaFormatReader::GetBuffered()
     intervals = Move(videoti);
   }
 
+  if (!intervals.Length() ||
+      intervals.GetStart() == media::TimeUnit::FromMicroseconds(0)) {
+    // IntervalSet already starts at 0 or is empty, nothing to shift.
+    return intervals;
+  }
   return intervals.Shift(media::TimeUnit::FromMicroseconds(-startTime));
 }