Temporarily remove MeasureAs= for File#lastModifiedDate as it crashes in workers

I'll take a look not to make MeasureAs crash in workers in a follow-up patch.

BUG=330665
TEST=fast/files/workers/worker-file-last-modified.html

Review URL: https://codereview.chromium.org/98183013

git-svn-id: svn://svn.chromium.org/blink/trunk@164496 bbb929c8-8fbe-4397-9dbb-9b2b20218538