Revert of HTMLPlugInElement: Use custom focus logic only when there is a plugin. (patchset #1 id:1 of https://codereview.chromium.org/717193003/)

Reason for revert:
probable cause of use-after-free:
https://code.google.com/p/chromium/issues/detail?id=433357

Original issue's description:
> HTMLPlugInElement: Use custom focus logic only when there is a plugin.
> 
> This allows focus to enter and leave plugin placeholders, so that constituent
> elements (such as a close button) can be keyboard-focused.
> 
> TEST=fast/plugins/plugin/placeholder-focus.html
> BUG=364716
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=185290

TBR=mkwst@chromium.org,esprehn@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=364716

Review URL: https://codereview.chromium.org/729573002

git-svn-id: svn://svn.chromium.org/blink/trunk@185383 bbb929c8-8fbe-4397-9dbb-9b2b20218538