Communicate seeking state to controls.

Regression from fcbb725. Other elements rely on controls
knowing current seek state. Now that seek bar is its own
class, it needs to update controls on that.

Change-Id: I29ae8faa9f830ef86380d6b07373dedbeebce263

ui/controls.js

@@ -491,6 +491,15 @@ shaka.ui.Controls = class extends shaka.util.FakeEventTarget {
     return this.isSeeking_;
   }
 
+
+  /**
+   * @param {boolean} seeking
+   * @export
+   */
+  setSeeking(seeking) {
+    this.isSeeking_ = seeking;
+  }
+
   /**
    * @return {boolean}
    * @export

ui/seek_bar.js

@@ -86,6 +86,7 @@ shaka.ui.SeekBar = class extends shaka.ui.RangeElement {
    * @override
    */
   onChangeStart() {
+    this.controls.setSeeking(true);
     this.video.pause();
   }
 
@@ -126,7 +127,7 @@ shaka.ui.SeekBar = class extends shaka.ui.RangeElement {
     // They just let go of the seek bar, so cancel the timer and manually
     // call the event so that we can respond immediately.
     this.seekTimer_.tickNow();
-
+    this.controls.setSeeking(false);
     this.video.play();
   }