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

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

Index: mojo/public/interfaces/bindings/tests/test_constants.mojom
diff --git a/mojo/public/interfaces/bindings/tests/test_constants.mojom b/mojo/public/interfaces/bindings/tests/test_constants.mojom
index 462d5126a5a8d76b9292ccc3c678eb872bb6f661..272e6035a6fed1842a4d1eb0789a70c5c2f769e8 100644
--- a/mojo/public/interfaces/bindings/tests/test_constants.mojom
+++ b/mojo/public/interfaces/bindings/tests/test_constants.mojom
@@ -42,12 +42,16 @@ 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";
 };