Mark plugins/mouse-click-plugin-clears-selection.html as failing

TBR=jochen
BUG=260351

Review URL: https://codereview.chromium.org/20128002

git-svn-id: svn://svn.chromium.org/blink/trunk@154845 bbb929c8-8fbe-4397-9dbb-9b2b20218538