Add with_statement import for python2.5.
Add with_statement import for python2.5.

See http://www.python.org/dev/peps/pep-0343/ which describes
the with statement.
Review URL: http://codereview.chromium.org/5690003
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp