Fix the hasBackingFile flag for File backed by a file system URL.

We assume File instance has a file path if hasBackiingFile() is true.
But previously the constructor for file system URL specifies true to the flag.
This CL turns the flag to false at the constructor.

BUG=NONE

Review URL: https://codereview.chromium.org/660763004

git-svn-id: svn://svn.chromium.org/blink/trunk@183871 bbb929c8-8fbe-4397-9dbb-9b2b20218538