Skip _CheckFilePermissions check on Windows

Executable bits are wonky on Windows, so checking them doesn't get us anything
and incorrectly disallows uploading of, e.g., python scripts.

This is the Blink equivalent to https://codereview.chromium.org/142973008/

R=thakis@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/151843006

git-svn-id: svn://svn.chromium.org/blink/trunk@166338 bbb929c8-8fbe-4397-9dbb-9b2b20218538