chromium/android_webview/tools/system_webview_shell/apk/res/layout/activity_web_platform_tests.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/rootLayout"
    android:weightSum="1">
  <WebView
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:id="@+id/rootWebView"
      android:layout_gravity="center_horizontal"
      android:layout_alignParentStart="true"
      android:layout_marginStart="0dp"
      />
  <!-- All child layouts will be inserted here linearly. -->
</RelativeLayout>