Add proguard flags for JUnit4 tests

testing/android/proguard_for_test.flags

Index: testing/android/proguard_for_test.flags
diff --git a/testing/android/proguard_for_test.flags b/testing/android/proguard_for_test.flags
index 3326074db6ab3d562af70be4951741d57bab9b33..51f1428772d9effee7f7155da3273000b19ee666 100644
--- a/testing/android/proguard_for_test.flags
+++ b/testing/android/proguard_for_test.flags
@@ -7,11 +7,22 @@
 # 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