removeChipOnBackspace_() {
    if (this.isLocked_) {
      return;
    }
    if (this.shadowInputElem_.value.length == 0 && this.getChildCount() > 0) {
      var chip = this.getChildAt(this.getChildCount() - 1);
      this.removeChild(chip, true);
    }

    if (this.getChildCount() == 0) {
      goog.style.setWidth(this.shadowInputElem_, 100);
    }
  }
Removes the last chip from the holder if the input field is empty when the
user has pressed backspace. Aborts if ChipHolder is locked.
@private