Use kPointerSize to compute the call instruction offset in the PatchedReturnSequence for X64

BUG=
R=danno@chromium.org

Review URL: https://codereview.chromium.org/19763010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15764 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/x64/assembler-x64-inl.h

@@ -369,7 +369,7 @@ bool RelocInfo::IsPatchedReturnSequence() {
   // The 11th byte is int3 (0xCC) in the return sequence and
   // REX.WB (0x48+register bit) for the call sequence.
 #ifdef ENABLE_DEBUGGER_SUPPORT
-  return pc_[10] != 0xCC;
+  return pc_[2 + kPointerSize] != 0xCC;
 #else
   return false;
 #endif