Reuse ui_test_pak target for aura's TestAuraInitializer.

Instead of duplicating the same logic over and over everywhere, just
make use of the ui_test_pak that was created for that purpose.

BUG=None
TEST=content_unittests
R=tony@chromium.org
TBR=ben

Review URL: https://codereview.chromium.org/97663002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238564 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238564