Mojo C++ bindings: support mapping mojo string to WTF::String.

mojo/public/interfaces/bindings/tests/BUILD.gn

Index: mojo/public/interfaces/bindings/tests/BUILD.gn
diff --git a/mojo/public/interfaces/bindings/tests/BUILD.gn b/mojo/public/interfaces/bindings/tests/BUILD.gn
index 42f16c33e4733dd2c6f1e740afea71c7fb76aacd..71b1a64c98daa42d0e97d9dbcf0f1c27cfaef004 100644
--- a/mojo/public/interfaces/bindings/tests/BUILD.gn
+++ b/mojo/public/interfaces/bindings/tests/BUILD.gn
@@ -114,3 +114,30 @@ mojom("test_interfaces_blink") {
     ":test_interfaces",
   ]
 }
+
+mojom("test_wtf_types") {
+  testonly = true
+
+  sources = [
+    "test_wtf_types.mojom",
+  ]
+
+  with_environment = false
+}
+
+mojom("test_wtf_types_wtf") {
+  testonly = true
+
+  sources = [
+    "test_wtf_types.mojom",
+  ]
+
+  deps = [
+    ":test_wtf_types",
+  ]
+
+  use_wtf_types = true
+  variant = "wtf"
+
+  with_environment = false
+}