Add an optional source length field to the Extension constructor.

src/heap.cc

Index: src/heap.cc
===================================================================
--- src/heap.cc	(revision 9266)
+++ src/heap.cc	(working copy)
@@ -2805,7 +2805,7 @@
 
 
 MaybeObject* Heap::AllocateExternalStringFromAscii(
-    ExternalAsciiString::Resource* resource) {
+    const ExternalAsciiString::Resource* resource) {
   size_t length = resource->length();
   if (length > static_cast<size_t>(String::kMaxLength)) {
     isolate()->context()->mark_out_of_memory();
@@ -2828,7 +2828,7 @@
 
 
 MaybeObject* Heap::AllocateExternalStringFromTwoByte(
-    ExternalTwoByteString::Resource* resource) {
+    const ExternalTwoByteString::Resource* resource) {
   size_t length = resource->length();
   if (length > static_cast<size_t>(String::kMaxLength)) {
     isolate()->context()->mark_out_of_memory();