execfile() does not exist in Python 3; replace it with open(), read()
and exec().

Bug: 984182
Change-Id: I2266ff2bd63bffc458bd04500c8c0d6c360f1b4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1844831
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>