Implement File constructor.

The feature is behind the FileConstructor RuntimeEnabledFeature with
status=experimental.

BUG=164933

Review URL: https://codereview.chromium.org/57483002

git-svn-id: svn://svn.chromium.org/blink/trunk@162116 bbb929c8-8fbe-4397-9dbb-9b2b20218538