Rename wm_core_export.h to wm_export.h and the wm_core target to just wm

Extracted from https://codereview.chromium.org/209383003/

http://crbug.com/308710
TBR=sky@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=263812

Review URL: https://codereview.chromium.org/237873003

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@263994 4ff67af0-8c30-449e-8e8b-ad334ec8d88c