You can do it using FrameLayout
as follows...
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<WebView
android:id="@+id/webview"
android:layout_width="fill_parent"
android:layout_height="fill_parent" />
<Button
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_gravity="bottom"
android:text="Button" />
</FrameLayout>