Fix base::internal::Singleton to use acquire loads instead of nobarrier ones.
The NoBarrier_Load operations don't actually guarantee that the singleton data
associated with the pointer is visible to the thread calling get() even if the
pointer is initialized.

R=mark@chromium.org, dvyukov@chromium.org, epenner@chromium.org

Review URL: https://codereview.chromium.org/188603002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255552 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255552