Fix build with new googletest

The new googletest requires prefix to be non-empty.

Exempt-From-Owner-Approval: Fix test
Change-Id: I69505920cc2c12abd8214210918a7bbfea431ea7
diff --git a/base/message_loop/message_loop_unittest.cc b/base/message_loop/message_loop_unittest.cc
index 7743479..6cf6bc3 100644
--- a/base/message_loop/message_loop_unittest.cc
+++ b/base/message_loop/message_loop_unittest.cc
@@ -1774,7 +1774,7 @@
 }
 
 INSTANTIATE_TEST_CASE_P(
-    ,
+    All,
     MessageLoopTypedTest,
     ::testing::Values(MessageLoopTypedTestParams(
                           MessageLoop::TYPE_DEFAULT,
@@ -2227,7 +2227,7 @@
 }
 
 INSTANTIATE_TEST_CASE_P(
-    ,
+    All,
     MessageLoopTest,
     ::testing::Values(TaskSchedulerAvailability::NO_TASK_SCHEDULER
                       // Unsupported in libchrome