Mojo C++ bindings: introduce mojo::WTFMap for blink bindings.

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

Index: mojo/public/interfaces/bindings/tests/test_structs.mojom
diff --git a/mojo/public/interfaces/bindings/tests/test_structs.mojom b/mojo/public/interfaces/bindings/tests/test_structs.mojom
index f16bd6654293d617292645b4c80f0a9583c7f49d..def4f1db8deb5144d495efe96e600e1f28f09bd6 100644
--- a/mojo/public/interfaces/bindings/tests/test_structs.mojom
+++ b/mojo/public/interfaces/bindings/tests/test_structs.mojom
@@ -113,7 +113,8 @@ struct ScopedConstants {
 // decoded successfully.
 
 struct MapKeyTypes {
-  map<bool, bool> f0;
+  // TODO(yzshen): WTF::HashMap doesn't support bool as key.
+  // map<bool, bool> f0;
   map<int8, int8> f1;
   map<uint8, uint8> f2;
   map<int16, int16> f3;