Make mojom-generated C++ string constants really constant.

mojo/public/interfaces/bindings/tests/test_constants.mojom

 // Copyright 2015 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.
 
 [JavaPackage="org.chromium.mojo.bindings.test.mojom.test_constants"]
 module mojo.test;
 
 // Integral types.
 const bool kBoolValue = true;
 
@@ skipping 24 matching lines @@
 const double kDoubleValue = 3.14159;
 const double kDoubleInfinity = double.INFINITY;
 const double kDoubleNegativeInfinity = double.NEGATIVE_INFINITY;
 const double kDoubleNaN = double.NAN;
 
 const float kFloatValue = 2.71828;
 const float kFloatInfinity = float.INFINITY;
 const float kFloatNegativeInfinity = float.NEGATIVE_INFINITY;
 const float kFloatNaN = float.NAN;
 
+const string kStringValue = "test string contents";
+
 struct StructWithConstants {
   const int8 kInt8Value = 5;
   const float kFloatValue = 765.432;
+  const string kStringValue = "struct test string contents";
 };
 
 interface InterfaceWithConstants {
   const uint32 kUint32Value = 20100722;
   const double kDoubleValue = 12.34567;
+  const string kStringValue = "interface test string contents";
 };