Bug 1385699. P3 - remove MediaDecoder::SetInfinite() and related code. r=cpearce

Since inf can be encoded in MDSM::mDuration, we don't need an additional flag
in MediaDecoder to indicate 'infinite' anymore. Note duration change from infinite
to finite is handled by MDSM, so we can remove the explicit calls to SetInfinite(false).

MozReview-Commit-ID: EoxwZJzPAJl

dom/html/HTMLMediaElement.cpp

@@ -5505,7 +5505,7 @@ void HTMLMediaElement::PlaybackEnded()
     }
   }
 
-  if (mSrcStream || (mDecoder && mDecoder->IsInfinite())) {
+  if (mSrcStream) {
     LOG(LogLevel::Debug, ("%p, got duration by reaching the end of the resource", this));
     DispatchAsyncEvent(NS_LITERAL_STRING("durationchange"));
   }