Bug 1382780 - part2 : separate the value of actual playback rate and media element's attribute. r=cpearce

According to [1], we should separate the value of actual playback rate and media elemenet's attribute.
But still keep a MAX and MIN threshold to control the highest and lowser playback rate for decoder.

[1] http://w3c-test.org/html/semantics/embedded-content/media-elements/playing-the-media-resource/playbackRate.html

MozReview-Commit-ID: EGCa4yrNXO6

--HG--
extra : rebase_source : d03b831eeebb3e3e3bb0759394eab0116be0f1d5

dom/html/HTMLMediaElement.cpp

@@ -6723,7 +6723,11 @@ HTMLMediaElement::SetPlaybackRate(double aPlaybackRate, ErrorResult& aRv)
     return;
   }
 
-  mPlaybackRate = ClampPlaybackRate(aPlaybackRate);
+  if (mPlaybackRate == aPlaybackRate) {
+    return;
+  }
+
+  mPlaybackRate = aPlaybackRate;
 
   if (mPlaybackRate != 0.0 &&
       (mPlaybackRate > THRESHOLD_HIGH_PLAYBACKRATE_AUDIO ||
@@ -6734,7 +6738,7 @@ HTMLMediaElement::SetPlaybackRate(double aPlaybackRate, ErrorResult& aRv)
   }
 
   if (mDecoder) {
-    mDecoder->SetPlaybackRate(mPlaybackRate);
+    mDecoder->SetPlaybackRate(ClampPlaybackRate(mPlaybackRate));
   }
   DispatchAsyncEvent(NS_LITERAL_STRING("ratechange"));
 }