UseCounter for File.lastModifiedDate.

The latest ED for the File API prescribes a lastModified attribute that
returns a number and removes the lastModifiedDate that returned a date.
This will help us decide if we can deprecate and remove the old
attribute.

TEST=Chromium compiles.
BUG=

Review URL: https://codereview.chromium.org/105763002

git-svn-id: svn://svn.chromium.org/blink/trunk@163211 bbb929c8-8fbe-4397-9dbb-9b2b20218538