GTK build fix: Have a dummy aura_builder target even with use_aura=0.

BUG=none
TBR=erg@chromium.org

Review URL: https://codereview.chromium.org/131453013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249176 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249176