Add a CheckFilePermissions to Blink's PRESUBMIT

This uses the same presubmit check used in Chromium, based on checkperms.py,
which disallows executable bits on files that shouldn't be executable (such
as cpp, h, js, html files).

R=thakis@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/132233062

git-svn-id: svn://svn.chromium.org/blink/trunk@166077 bbb929c8-8fbe-4397-9dbb-9b2b20218538