Correct condition and flag state for skipping PLAY request.

* When PLAY request for casted video is skipped, this.playInProgress_ was not cleared and this inconsistent state prevented the video from being played. I cleared the flag in this case.
* When PAUSE reqeust is in progress, incoming PLAY request should not be skipped. So I added a condition to skip the PLAY request.

BUG=417520,417538
TEST=manually tested the steps described in both bugs.

Review URL: https://codereview.chromium.org/603413003

Cr-Commit-Position: refs/heads/master@{#297137}

ui/file_manager/video_player/js/cast/cast_video_element.js

@@ -224,8 +224,12 @@ CastVideoElement.prototype = {
       return;
 
     var play = function() {
+      // If the casted media is already playing and a pause request is not in
+      // progress, we can skip this play request.
       if (this.castMedia_.playerState ===
-              chrome.cast.media.PlayerState.PLAYING) {
+              chrome.cast.media.PlayerState.PLAYING &&
+          !this.pauseInProgress_) {
+        this.playInProgress_ = false;
         return;
       }