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 android testing support lib

[jbudorick, 2017/03/22 00:17:20]

Why do we need these two additions?

[the real yoland, 2017/03/22 00:28:15]

AndroidJUnitRunner reflectively calls for class runner's constructor, without
this one, runner would complain class runner class
is not found

Also, if the exception can match accurately to the line number, it makes
debugging easier.

+-keep class * extends android.support.test.** {
+    *;
+}
+
+# Keep any class inherited from junit 4 library
+-keep class * extends org.junit.** {
+    *;
+}
+
 # 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.** {
   *;
 }