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

mojo/public/cpp/bindings/BUILD.gn

Index: mojo/public/cpp/bindings/BUILD.gn
diff --git a/mojo/public/cpp/bindings/BUILD.gn b/mojo/public/cpp/bindings/BUILD.gn
index b17262133d0006e80aa8dd18ba307e4894fd8ae4..9e166794ac61e824db56df1836354f1bd15c4242 100644
--- a/mojo/public/cpp/bindings/BUILD.gn
+++ b/mojo/public/cpp/bindings/BUILD.gn
@@ -80,6 +80,10 @@ source_set("bindings") {
     "lib/validation_util.cc",
     "lib/validation_util.h",
     "lib/value_traits.h",
+
+    # Include the .h but not the .cc file. The .h file is used by
+    # serialization_forward.h.
+    "lib/wtf_string_serialization.h",
     "map.h",
     "message.h",
     "message_filter.h",
@@ -120,3 +124,18 @@ source_set("callback") {
     "//mojo/public/cpp/system",
   ]
 }
+
+source_set("wtf_support") {
+  sources = [
+    "lib/wtf_serialization.h",
+    "lib/wtf_string_serialization.cc",
+    "lib/wtf_string_serialization.h",
+  ]
+
+  deps = [
+    ":bindings",
+    "//third_party/WebKit/Source/wtf",
+  ]
+
+  public_configs = [ "//third_party/WebKit/Source:config" ]
+}