Center ProgressBar

I want to full center a progressbar with a full height/width webview, and i'm not able to. Here is my xml code:

<?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:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.stevejobs.iphone.MainActivity">

    <WebView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/webview"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true">

        <ProgressBar
            style="?android:attr/progressBarStyle"
            android:id="@+id/progressBar"
            android:layout_width="150dp"
            android:layout_height="150dp"
            android:layout_centerHorizontal="true"
            android:layout_centerVertical="true"
            android:layout_centerInParent="true"
            android:foregroundGravity="center"/>
    </WebView>
</RelativeLayout>

Am I missing any line here to center it? The progressBar appears on te top left corner right now.

try this

  <?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:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.stevejobs.iphone.MainActivity">

    <WebView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/webview"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"/>

   <ProgressBar
            style="?android:attr/progressBarStyle"
            android:id="@+id/progressBar"
            android:layout_width="150dp"
            android:layout_height="150dp"
            android:layout_centerHorizontal="true"
            android:layout_centerVertical="true"
            android:layout_centerInParent="true"
            android:foregroundGravity="center"/>
</RelativeLayout>