Add helpers for logging mojom objects.

mojo/public/cpp/bindings/tests/string_unittest.cc

 // Copyright 2014 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.
 
+#include <sstream>
+
 #include "mojo/public/cpp/bindings/string.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace mojo {
 namespace test {
 
 TEST(StringTest, DefaultIsNull) {
   String s;
   EXPECT_TRUE(s.is_null());
 }
@@ skipping 94 matching lines @@
   String null;
   String null2;
   EXPECT_FALSE(null < null2);
   EXPECT_FALSE(null2 < null);
 
   String real("real");
   EXPECT_TRUE(null < real);
   EXPECT_FALSE(real < null);
 }
 
+TEST(StringText, OutputFormatting) {
+  String s("abc");
+  String null;
+
+  std::ostringstream so;
+  so << "s=" << s << ", null=" << null;
+  EXPECT_EQ("s=abc, null=", so.str());
+}
+
 }  // namespace test
 }  // namespace mojo