Reset all EventHandler data members in EventHandler::clear()

All members are reset with the exception of m_frame which is the pointer to
the owner frame. This ensures any residual state is cleared.

  
BUG=270123

Review URL: https://codereview.chromium.org/643143003

git-svn-id: svn://svn.chromium.org/blink/trunk@185062 bbb929c8-8fbe-4397-9dbb-9b2b20218538