Add output formatters for more mojom types.

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

Index: mojo/public/cpp/bindings/tests/string_unittest.cc
diff --git a/mojo/public/cpp/bindings/tests/string_unittest.cc b/mojo/public/cpp/bindings/tests/string_unittest.cc
index 0769071b15de760fae1b2d11b33fa4b76d2bf754..4089039bda38e849a76d63d473b8498957d28ac9 100644
--- a/mojo/public/cpp/bindings/tests/string_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/string_unittest.cc
@@ -2,6 +2,8 @@
 // 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"
 
@@ -116,5 +118,14 @@ TEST(StringTest, LessThanNullness) {
   EXPECT_FALSE(real < null);
 }
 
+TEST(StringTest, 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