plugins/plugin-document-back-forward.html is flaky

BUG=340150
TBR=caseq@chromium.org

Review URL: https://codereview.chromium.org/152413002

git-svn-id: svn://svn.chromium.org/blink/trunk@166276 bbb929c8-8fbe-4397-9dbb-9b2b20218538