Split app_list test support files into its own specific target.

This creates a separate target named 'app_list_test_support', that is
independent from ui_unittests.gyp. This is necessary because we are in
the process of moving ui_unittests.gyp into ui/base, and we will split
ui_unittests into multiple module specific unit tests.

BUG=299841
TEST=None
R=ben@chromium.org,xiyuan@chromium.org

Review URL: https://codereview.chromium.org/80803003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236740 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236740