Add an optional source length field to the Extension constructor.

src/heap.h

Index: src/heap.h
===================================================================
--- src/heap.h	(revision 9266)
+++ src/heap.h	(working copy)
@@ -796,9 +796,9 @@
   // failed.
   // Please note this does not perform a garbage collection.
   MUST_USE_RESULT MaybeObject* AllocateExternalStringFromAscii(
-      ExternalAsciiString::Resource* resource);
+      const ExternalAsciiString::Resource* resource);
   MUST_USE_RESULT MaybeObject* AllocateExternalStringFromTwoByte(
-      ExternalTwoByteString::Resource* resource);
+      const ExternalTwoByteString::Resource* resource);
 
   // Finalizes an external string by deleting the associated external
   // data and clearing the resource pointer.