Deflake fast/dom/Window/window-postmessage-clone.html

Credit goes to Alexey Proskuryakov for fixing this in WebKit:
http://trac.webkit.org/r155725

Turns out, the lastModifiedDate of non-existent file is current time,
which is only current at one moment. If you expect two different times
to be both current, you're going to have a bad time.

TBR=ojan
BUG=

Review URL: https://codereview.chromium.org/26437003

git-svn-id: svn://svn.chromium.org/blink/trunk@159135 bbb929c8-8fbe-4397-9dbb-9b2b20218538