linux_aura: Fix cursor memeory leak, which was redding the lsan bot.

views::corewm::CursorManager was no longer owned by anything, nor did it
register with the base::AtExitManager. This refcounts the number of DNWAs
and frees the shared CursorManager when the last one is destroyed. (We
can't just use a base::LazyInstance<> since that doesn't have any useful
guarantees on destruction ordering; we need to destroy CursorManager
before we tear down the X11 connection.)

BUG=334333
R=tdanderson
TBR=sadrul

Review URL: https://codereview.chromium.org/134103006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245090 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245090