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

mojo/public/tools/bindings/mojom.gni

Index: mojo/public/tools/bindings/mojom.gni
diff --git a/mojo/public/tools/bindings/mojom.gni b/mojo/public/tools/bindings/mojom.gni
index 6e2a62a8aa0fc4c8102bdd9395d6f0b7ff541ca5..abd9f28863f6407733cb24849e638653844229e1 100644
--- a/mojo/public/tools/bindings/mojom.gni
+++ b/mojo/public/tools/bindings/mojom.gni
@@ -64,6 +64,9 @@ mojom_generator_sources = [
 #   testonly (optional)
 #
 #   visibility (optional)
+#
+#   use_wtf_types (optional, C++ only)

[Ken Rockot(use gerrit already), 2016/03/01 02:06:37]

nit: How about more generically something like for_blink? I would like to use
the same flag to generate blink JS bindings eventually

[yzshen1, 2016/03/01 18:42:46]

Sounds good. Done.

+#       Use WTF types as generated type for mojo string/array/map.
 template("mojom") {
   assert(
       defined(invoker.sources) || defined(invoker.deps) ||
@@ -163,6 +166,10 @@ template("mojom") {
         }
         inputs += invoker.typemaps
       }
+
+      if (defined(invoker.use_wtf_types) && invoker.use_wtf_types) {
+        args += [ "--use_wtf_types" ]
+      }
     }
   }
 
@@ -253,6 +260,12 @@ template("mojom") {
       if (!defined(invoker.with_environment) || invoker.with_environment) {
         deps += [ "//mojo/environment:chromium" ]
       }
+      if (defined(invoker.use_wtf_types) && invoker.use_wtf_types) {
+        public_deps += [
+          "//mojo/public/cpp/bindings:wtf_support",
+          "//third_party/WebKit/Source/wtf",
+        ]
+      }
     }
   }