Const-corrected some MachineSignature* parameters.

src/compiler/x64/linkage-x64.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/assembler.h"
 #include "src/code-stubs.h"
 #include "src/compiler/linkage.h"
 #include "src/compiler/linkage-impl.h"
@@ skipping 56 matching lines @@
 CallDescriptor* Linkage::GetStubCallDescriptor(
     Isolate* isolate, Zone* zone, const CallInterfaceDescriptor& descriptor,
     int stack_parameter_count, CallDescriptor::Flags flags,
     Operator::Properties properties) {
   return LH::GetStubCallDescriptor(isolate, zone, descriptor,
                                    stack_parameter_count, flags, properties);
 }
 
 
 CallDescriptor* Linkage::GetSimplifiedCDescriptor(Zone* zone,
-                                                  MachineSignature* sig) {
+                                                  const MachineSignature* sig) {
   return LH::GetSimplifiedCDescriptor(zone, sig);
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8