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 3c1520a679e465de6335c5f35408862294b1c120..2b1f461d28d38469e1ac8563978801fabb90d540 100644
--- a/mojo/public/cpp/bindings/string.h
+++ b/mojo/public/cpp/bindings/string.h
@@ -25,6 +25,9 @@ class String {
   }
   String(const char* chars, size_t num_chars)
       : 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) {}
@@ -39,6 +42,11 @@ 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;
@@ -115,6 +123,18 @@ 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 <>