ContentLoadingProgressBar不显示问题

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.leo.enjoytime.activity.WebViewActivity">

    <WebView
        android:id="@+id/web_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

    <android.support.v4.widget.ContentLoadingProgressBar
        android:id="@+id/progressbar"
        style="?android:attr/android:progressBarStyleHorizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        />

</RelativeLayout>