Add ScopedVector<T>::pop_back().

TBR=willchan

Review URL: https://codereview.chromium.org/96553002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237921 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237921