Extract hardcoded error strings into a single place and replace them with enum.

test/cctest/test-code-stubs-x64.cc

Index: test/cctest/test-code-stubs-x64.cc
diff --git a/test/cctest/test-code-stubs-x64.cc b/test/cctest/test-code-stubs-x64.cc
index e30c160e7ab08d6bce785a42e8cfcf242bab3e10..0bffb87fef03fda8e88e27c7587ed4656897152e 100644
--- a/test/cctest/test-code-stubs-x64.cc
+++ b/test/cctest/test-code-stubs-x64.cc
@@ -93,7 +93,7 @@ ConvertDToIFunc MakeConvertDToIFuncTrampoline(Isolate* isolate,
     Register reg = Register::from_code(reg_num);
     if (!reg.is(rsp) && !reg.is(rbp) && !reg.is(destination_reg)) {
       __ cmpq(reg, MemOperand(rsp, 0));
-      __ Assert(equal, "register was clobbered");
+      __ Assert(equal, kRegisterWasClobbered);
       __ addq(rsp, Immediate(kPointerSize));
     }
   }