[VSAddin] Don't require both 2010 and 2012 to be installed.

R=binji@chromium.org

Review URL: https://codereview.chromium.org/153163002

git-svn-id: https://nativeclient-sdk.googlecode.com/svn/trunk/src@1570 050acbb0-2703-11df-ab0a-9f3f633ae91d