Add an optional source length field to the Extension constructor.

src/heap.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 778 matching lines @@
       int start,
       int end,
       PretenureFlag pretenure = NOT_TENURED);
 
   // Allocate a new external string object, which is backed by a string
   // resource that resides outside the V8 heap.
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
   // 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.
   inline void FinalizeExternalString(String* string);
 
   // Allocates an uninitialized object.  The memory is non-executable if the
   // hardware and OS allow.
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
   // failed.
   // Please note this function does not perform a garbage collection.
@@ skipping 1537 matching lines @@
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG || LIVE_OBJECT_LIST
 
 
 } }  // namespace v8::internal
 
 #undef HEAP
 
 #endif  // V8_HEAP_H_