mojom: Add associative arrays to the mojom language.

mojo/public/cpp/bindings/string.h

Index: mojo/public/cpp/bindings/string.h
diff --git a/mojo/public/cpp/bindings/string.h b/mojo/public/cpp/bindings/string.h
index cd7db26e8230a8f1fa1df62822e3d972bb60e571..ef4f2b68163392de684df883556dd2a912862533 100644
--- a/mojo/public/cpp/bindings/string.h
+++ b/mojo/public/cpp/bindings/string.h
@@ -27,6 +27,10 @@ class String {
       : value_(chars, num_chars),
         is_null_(false) {
   }
+  String(const mojo::String& str)
+      : value_(str.value_),
+        is_null_(str.is_null_) {
+  }
   template <size_t N>
   String(const char chars[N]) : value_(chars, N-1), is_null_(false) {}
 
@@ -40,6 +44,11 @@ class String {
     return TypeConverter<U, String>::Convert(*this);
   }
 
+  String& operator=(const mojo::String& str) {
+    value_ = str.value_;
+    is_null_ = str.is_null_;
+    return *this;
+  }
   String& operator=(const std::string& str) {
     value_ = str;
     is_null_ = false;
@@ -110,6 +119,18 @@ inline std::ostream& operator<<(std::ostream& out, const String& s) {
   return out << s.get();
 }
 
+inline bool operator<(const String& a, const String& b) {
+  if (a.is_null() && !b.is_null()) {
+    // The null string is less than a non-NULL string.
+    return true;
+  } else if (a.is_null() || b.is_null()) {
+    // in all other cases where A or B is NULL, |a| is not less than |b|.
+    return false;
+  }
+
+  return a.get() < b.get();
+}
+
 // TODO(darin): Add similar variants of operator<,<=,>,>=
 
 template <>