Deprecate named base::SharedMemory.

      It's hardly used and not implemented on various platforms. (More deprecation
work here to come.)

R=brettw@chromium.org
TBR=sky@chromium.org
BUG=345734

Review URL: https://codereview.chromium.org/186473002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255148 0039d316-1c4b-4281-b951-d872f2087c98