Changed all instances of 'packaged apps' or 'packaged app' to Chrome app or Chrome apps.

This will hopefully be a small, but effective SEO step.

R=miket@chromium.org

Review URL: https://codereview.chromium.org/27250002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228784 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228784