Make cloning of File objects more useful.

Make the structured cloning of File objects complete, extending their
wire format to also include:

 * name
 * relative path
 * size
 * last modification date.

As this changes the format, bump the serialization version number up
to 4. The deserializer is backwards compatible, including for File
objects; verify this to be so through tests.

Files constructed via the File constructor weren't correctly cloned
as-was; added a Worker test to have some coverage.

R=dslomov@chromium.org,jochen@chromium.org
BUG=171905
TEST=fast/storage/serialized-script-value
TEST=fast/files/workers/worker-read-file-constructor-async

Review URL: https://codereview.chromium.org/100023005

git-svn-id: svn://svn.chromium.org/blink/trunk@163956 bbb929c8-8fbe-4397-9dbb-9b2b20218538