Move wm/core to wm namespace.

Also sets up new targets for wm_core, wm_core_unittests, etc.

R=sky@chromium.org
TBR=sky@chromium.org
BUG=

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=256680

Review URL: https://codereview.chromium.org/196063002

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@256839 4ff67af0-8c30-449e-8e8b-ad334ec8d88c