Clean up common.gypi.

This removes a handful of redundant variables and GYP includes.

BUG=
R=mark@chromium.org

Review URL: https://breakpad.appspot.com/2654002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1321 4c0a9323-5329-0410-9bdc-e9ce6186880e