Add an optional source length field to the Extension constructor.

src/factory.h

Index: src/factory.h
===================================================================
--- src/factory.h	(revision 9266)
+++ src/factory.h	(working copy)
@@ -145,9 +145,9 @@
   // not make sense to have a UTF-8 factory function for external strings,
   // because we cannot change the underlying buffer.
   Handle<String> NewExternalStringFromAscii(
-      ExternalAsciiString::Resource* resource);
+      const ExternalAsciiString::Resource* resource);
   Handle<String> NewExternalStringFromTwoByte(
-      ExternalTwoByteString::Resource* resource);
+      const ExternalTwoByteString::Resource* resource);
 
   // Create a global (but otherwise uninitialized) context.
   Handle<Context> NewGlobalContext();