Add an optional source length field to the Extension constructor.

src/objects-inl.h

Index: src/objects-inl.h
===================================================================
--- src/objects-inl.h	(revision 9327)
+++ src/objects-inl.h	(working copy)
@@ -2376,25 +2376,27 @@
 }
 
 
-ExternalAsciiString::Resource* ExternalAsciiString::resource() {
+const ExternalAsciiString::Resource* ExternalAsciiString::resource() {
   return *reinterpret_cast<Resource**>(FIELD_ADDR(this, kResourceOffset));
 }
 
 
 void ExternalAsciiString::set_resource(
-    ExternalAsciiString::Resource* resource) {
-  *reinterpret_cast<Resource**>(FIELD_ADDR(this, kResourceOffset)) = resource;
+    const ExternalAsciiString::Resource* resource) {
+  *reinterpret_cast<const Resource**>(
+      FIELD_ADDR(this, kResourceOffset)) = resource;
 }
 
 
-ExternalTwoByteString::Resource* ExternalTwoByteString::resource() {
+const ExternalTwoByteString::Resource* ExternalTwoByteString::resource() {
   return *reinterpret_cast<Resource**>(FIELD_ADDR(this, kResourceOffset));
 }
 
 
 void ExternalTwoByteString::set_resource(
-    ExternalTwoByteString::Resource* resource) {
-  *reinterpret_cast<Resource**>(FIELD_ADDR(this, kResourceOffset)) = resource;
+    const ExternalTwoByteString::Resource* resource) {
+  *reinterpret_cast<const Resource**>(
+      FIELD_ADDR(this, kResourceOffset)) = resource;
 }