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..6bf0bbee973153ba2c2664291f88dcf76e352211 100644
--- a/testing/android/proguard_for_test.flags
+++ b/testing/android/proguard_for_test.flags
@@ -7,11 +7,26 @@
 # 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