Restrict multiple codesign assert.

A number of developers have found themselves in a state where they have multiple
copies of the same cert. Running `security find-identity -p codesigning`
should confirm this.  Instead, only assert if there identities with different
cert fingerprints.

BUG=None
R=sdefresne@chromium.org, thakis@chromium.org

Review URL: https://codereview.chromium.org/117463003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1821 78cadc50-ecff-11dd-a971-7dbc132099af