Deprecate base::SharedMemory's lock functionality.

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

Review URL: https://codereview.chromium.org/175183004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252933 0039d316-1c4b-4281-b951-d872f2087c98