Add chromium-junit4 tag to instrumentation test AndroidManifests.

webrtc/examples/androidtests/AndroidManifest.xml

 <!--
  *  Copyright 2017 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
 -->
 
 <manifest
       xmlns:android="http://schemas.android.com/apk/res/android"
       package="org.appspot.apprtc.test">
 
     <uses-permission android:name="android.permissions.RUN_INSTRUMENTATION" />
     <uses-sdk android:minSdkVersion="13" android:targetSdkVersion="21" />
 
     <uses-permission android:name="android.permission.RUN_INSTRUMENTATION" />
 
     <application>
         <uses-library android:name="android.test.runner" />
     </application>
 
+    <!-- Needed for chromium-junit4 tag. crbug.com/640116
+         TODO(sakal): Remove once the tag is no longer needed. -->
+    <!--suppress MissingPrefix -->
     <instrumentation android:name="android.support.test.runner.AndroidJUnitRunner"
         android:targetPackage="org.appspot.apprtc"
-        android:label="Tests for AppRTCMobile"/>
+        android:label="Tests for AppRTCMobile"
+        chromium-junit4="true"/>
 </manifest>