Sync am-string.

public/amtl/am-string.h

@@ -117,6 +117,8 @@ class AString
   }
 
  private:
+  static const size_t kInvalidLength = (size_t)-1;
+
   void set(const char *str, size_t length) {
     chars_ = new char[length + 1];
     length_ = length;