Valgrind: Suppress memory error in WebTestRunner::EventSender::reset.

BUG=309477
TBR=eae@chromium.org

Review URL: https://codereview.chromium.org/31253002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229708 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229708