Set NaN instead of 0 for null modificationTime

Set NaN instead of 0 for null modificationTime

As a part of multi-sided patches:
 http://codereview.chromium.org/10447110/ [landed]
 https://bugs.webkit.org/show_bug.cgi?id=87826

BUG=none
TEST=existing tests should pass

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=142131

[kinuko, 2012-06-08 12:04:19]

Tony, can you take a look?

[tony, 2012-06-08 18:32:14]

LGTM