Make setPlaybackRate behavior consistent. Closes #138.

Change-Id: I6a4be5acd37b42fb92bddc896009ae48cb92b5ec

lib/player/player.js

@@ -427,6 +427,13 @@ shaka.player.Player.prototype.onPlaying_ = function(event) {
   shaka.log.debug('onPlaying_', event);
   shaka.timer.begin('playing');
 
+  // Start rewind timer if playback rate should be negative and a rewind timer
+  // is not already set.
+  if (!this.rewindTimer_ && this.playbackRate_ < 0) {
+    this.video_.playbackRate = 0;
+    this.onRewindTimer_(
+        this.video_.currentTime, Date.now(), this.playbackRate_);
+  }
   if (this.buffering_) {
     this.endBufferingState_();
   }
@@ -673,7 +680,8 @@ shaka.player.Player.prototype.setPlaybackRate = function(rate) {
   if (rate >= 0) {
     // Slow-mo or fast-forward are handled natively by the UA.
     this.video_.playbackRate = rate;
-  } else {
+  // Only rewind when not paused.
+  } else if (!this.video_.paused) {
     // Rewind is not supported by any UA to date (2015), so we fake it.
     // http://crbug.com/33099
     this.video_.playbackRate = 0;