Remove 'using base::string16;' from base/strings/string16.h and remove temporary
PRESUBMIT.py _CheckForString16().

R=brettw@chromium.org, maruel@chromium.org
TBR=sky@chromium.org, tsepez@chromium.org
BUG=329295

Review URL: https://codereview.chromium.org/93903011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242733 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242733