[foozzie] Make Error immutable to prevent changes to stack-trace api

BUG=chromium:685633
NOTRY=true
TBR=yangguo@chromium.org, jarin@chromium.org

Review-Url: https://codereview.chromium.org/2658223002
Cr-Commit-Position: refs/heads/master@{#42726}

tools/foozzie/v8_mock.js

@@ -55,6 +55,7 @@ var __PrettyPrint = function __PrettyPrint(msg) { print(msg); };
 Error.prepareStackTrace = function (error, structuredStackTrace) {
   return "";
 };
+Object.freeze(Error)
 
 // Mock buffer access in float typed arrays because of varying NaN patterns.
 // Note, for now we just use noop forwarding proxies, because they already