Change event handling order in HTMLPlugInElement.

Currently, plugins receive events as a "default handler" when an
event is fired. "Default handlers" are called after the capturing and bubbling
phases. This means that there is no way for a plugin to stop propagation of an
event if it so desires.

This change causes plugins to receive events "at target" so they may stop
propagation of an event for the bubbling phase.

BUG=324351

Review URL: https://codereview.chromium.org/186153003

git-svn-id: svn://svn.chromium.org/blink/trunk@169059 bbb929c8-8fbe-4397-9dbb-9b2b20218538