Reset mouseCapture between tests if a mouse button wasn't released

Many tests call eventSender.mouseDown without then calling the
corresponding mouseUp method. For tests involving plugins we need to
reset the m_mouseCaptureNode between runs or events for the subsequent
test(s) will not be routed to the correct plugin instance.

BUG=305016
TEST=plugins/change-widget-and-click-crash.html plugins/plugin-initiate-popup-window.html
R=jsbell@chromium.org

Review URL: https://codereview.chromium.org/29023004

git-svn-id: svn://svn.chromium.org/blink/trunk@159980 bbb929c8-8fbe-4397-9dbb-9b2b20218538