Pass StatsTable shared memory via global descriptors on Posix rather than using named shared memory.

This is required for to enable chrome://stats Android where there is no
/dev/shm.  This also provides the added advantage of not requiring the
--no-sandbox command line flag with the --enable-stats-table on Posix.

BUG=None

Review URL: https://codereview.chromium.org/22911027

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228903 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228903