Fix update.py crash when GYP_DEFINES isn't present in the environment.

BUG=none
R=hans@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/158323004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250442 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250442