[foozzie] Fix Date proxy mocking

Fuzzers after switching to ToT mjsunit.js, which accesses
Date.prototype, while for correctness fuzzing it's mocked out as a
proxy.

This makes the proxy return the property correctly.

TBR=yangguo@chromium.org
NOTRY=true

Bug: chromium:813833
Change-Id: Ic4b6412063f42c4e75905fbd568b7f81bc54daff
Reviewed-on: https://chromium-review.googlesource.com/936046
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51539}

tools/clusterfuzz/v8_mock.js

@@ -69,6 +69,9 @@ var prettyPrinted = function prettyPrinted(msg) { return msg; };
       if (property == "now") {
         return mockDateNow;
       }
+      if (property == "prototype") {
+        return origDate.prototype
+      }
     },
   }