Add proguard flags for JUnit4 tests

testing/android/proguard_for_test.flags

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This file is for proguard flags which are applied to the combined test and
 # tested code. Do not put any flags in this file which might affect the
 # correctness of the .apk we are testing, since it will apply to that .apk as
 # well.
 
-# We want all tests to stick around.
+# Keep all junit3 tests
 -keep class * extends junit.framework.TestCase {
   *;
 }
 
+# Keep all junit4 tests
+-keep @**.RunWith class * {
+    *;
+}
+
+# Keep any class inherited from junit Runner since their constructor is called
+# reflectively
+-keep class * extends org.junit.runner.Runner {
+    *;
+}
+
 # TODO(yfriedman): Remove when crbug.com/488192 is fixed.
 -dontwarn org.apache.http.conn.scheme.LayeredSocketFactory
 
 # We have some "library class WebView depends on program class SslCertificate"
 # warnings, and they don't affect us.
 -dontwarn android.webkit.WebView*
 
 # We don't want BasicHttpParams or AbstractHttpParams to get renamed since that
 # then makes our calls to them use the implementation that we find in our .dex
 # file, which is broken. We need to rely on these calls resolving to the
 # system's implementation. See crbug.com/488192#c36.
 -keepnames class org.apache.** {
   *;
 }