Respsect "Last-Modified" for filesystem: URL too

Embedder can tell the resource's last-modified time, and
older static contents can be cached longer.
(This could affect existing apps, but hopefully not in a too
intrusive way)

Chromium side patch: https://codereview.chromium.org/137893004/

BUG=158861

Review URL: https://codereview.chromium.org/137583007

git-svn-id: svn://svn.chromium.org/blink/trunk@165143 bbb929c8-8fbe-4397-9dbb-9b2b20218538