[WIP] Initial CallFunctionCallback builtin.

src/assembler.h

Index: src/assembler.h
diff --git a/src/assembler.h b/src/assembler.h
index 06be048fa666e635a6c0a218b8704dff88155c70..da58a725e2764bbcf35f561e8d27c6b7809564cc 100644
--- a/src/assembler.h
+++ b/src/assembler.h
@@ -1089,6 +1089,13 @@ size_t hash_value(ExternalReference);
 
 V8_EXPORT_PRIVATE std::ostream& operator<<(std::ostream&, ExternalReference);
 
+inline int AddressOffset(ExternalReference ref0, ExternalReference ref1) {
+  ptrdiff_t offset = ref0.address() - ref1.address();
+  // Check that fits into int.
+  DCHECK_EQ(offset, static_cast<int>(offset));
+  return static_cast<int>(offset);
+}
+
 // -----------------------------------------------------------------------------
 // Utility functions