ui: Break the dependency on run_ui_unittests target.

This makes three unit tests targets stop depending on run_ui_unittests
target. By depending on it brings an indirect dependency on
chrome/chrome_resources.gyp:packed_resources. And we don't want that.
Instead we make these independent test targets load the test pak
themselves, so they can depend only in ui_test.pak.

BUG=144345
TEST=app_list_unittests, message_center_unittests, keyboard_unittests
R=tony@chromium.org,xiyuan@chromium.org,sadrul@chromium.org,miket@chromium.org

Review URL: https://codereview.chromium.org/143393003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246620 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246620