Fixed Gyp Xcode generator for libraries with identical names.

The Xcode generator produces the same hash for library target products that have
identical names and are defined in .gyp files with identical names that have
different source tree roots. The fix consists of setting the (already defined)
'projectRoot' property on the PBXProject object and adding it to the hashables
of this class.

Patch by Johannes Sartisohn <jsartisohn@google.com>

R=mark@chromium.org

Review URL: https://codereview.chromium.org/748793002

git-svn-id: http://gyp.googlecode.com/svn/trunk@2011 78cadc50-ecff-11dd-a971-7dbc132099af