Add #include <vector> to accelerators_gtk.h

This file uses std::vector but does not include corresponding header file.

R=erg@chromium.org
BUG=82098

Review URL: https://codereview.chromium.org/83293002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236823 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236823