Pull out Mac specific iPhoto and iTunes finding code

This is to enable mocking out the iTunes and iPhoto XML location for testing. 

BUG=NONE

Review URL: https://codereview.chromium.org/25389002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226950 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226950