Clean up HTMLPlugInElement.

- Fix style errors
- Wrap comments in 80 columns
- Shorten code
- Make the following members private:
 - DisplayState-related things
 - canContainRangeEndPoint
 - willRespondToMouseClickEvents
 - removeAllEventListeners
 - loadeMimeType
- Remove the following functions:
 - isCapturingMouseEvents
 - setIsCapturingMouseEvents
 - serviceType
 - loadedUrl

BUG=none
TEST=none; no behavior changes.
R=morrita@chromium.org, hayato@chromium.org

Review URL: https://codereview.chromium.org/42373002

git-svn-id: svn://svn.chromium.org/blink/trunk@160514 bbb929c8-8fbe-4397-9dbb-9b2b20218538