feat: Add a buffer fullness method (shaka-project#3392)

Closes shaka-project#3389

lib/player.js

@@ -2838,6 +2838,32 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
     return this.config_;
   }
 
+  /**
+   * Returns the ratio of video length buffered compared to buffering Goal
+   * @return {number}
+   * @export
+   */
+  getBufferFullness() {
+    if (this.video_) {
+      const bufferedLength = this.video_.buffered.length;
+      const bufferedEnd =
+          bufferedLength ? this.video_.buffered.end(bufferedLength - 1) : 0;
+      const bufferingGoal = this.getConfiguration().streaming.bufferingGoal;
+      const lengthToBeBuffered = Math.min(this.video_.currentTime +
+          bufferingGoal, this.seekRange().end);
+
+      if (bufferedEnd >= lengthToBeBuffered) {
+        return 1;
+      } else if (bufferedEnd <= this.video_.currentTime) {
+        return 0;
+      } else if (bufferedEnd < lengthToBeBuffered) {
+        return ((bufferedEnd - this.video_.currentTime) /
+            (lengthToBeBuffered - this.video_.currentTime));
+      }
+    }
+    return 0;
+  }
+
   /**
    * Reset configuration to default.
    * @export