onTouchStart_(event) {
    // Only proceed if controls are visible.
    if (!this.controls.isOpaque()) {
      return;
    }

    // If multiple touches, handle or ignore as needed. Here, we assume
    // single-touch.
    if (event.touches.length > 0) {
      this.touchStartX_ = event.touches[0].clientX;
      this.touchStartY_ = event.touches[0].clientY;
    }
    this.hasMoved_ = false;
  }
Called when the user starts touching the screen.
We record the initial touch coordinates for scroll detection.
@param {!TouchEvent} event
@private