Remove custom getters for File.lastModified{Date}

No particular need for these to be custom, so turned into File accessors
instead.

Also drop a runtime feature check for FileConstructor; it is now stable.

R=haraken
BUG=

Review URL: https://codereview.chromium.org/458953002

git-svn-id: svn://svn.chromium.org/blink/trunk@179944 bbb929c8-8fbe-4397-9dbb-9b2b20218538