Introduce ui/wm/ directory.

This starts by moving ui/shell into ui/wm/test/ directory and stabilish
a wm.gyp to hold all the targets for wm.

BUG=308710
TEST=None
R=ben@chromium.org

Review URL: https://codereview.chromium.org/74183002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235481 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235481