Add an optional source length field to the Extension constructor.

src/serialize.cc

Index: src/serialize.cc
===================================================================
--- src/serialize.cc	(revision 9327)
+++ src/serialize.cc	(working copy)
@@ -1420,7 +1420,7 @@
     if (!source->IsUndefined()) {
       ExternalAsciiString* string = ExternalAsciiString::cast(source);
       typedef v8::String::ExternalAsciiStringResource Resource;
-      Resource* resource = string->resource();
+      const Resource* resource = string->resource();
       if (resource == *resource_pointer) {
         sink_->Put(kNativesStringResource, "NativesStringResource");
         sink_->PutSection(i, "NativesStringResourceEnd");