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

Review URL: https://codereview.chromium.org/717193003

git-svn-id: svn://svn.chromium.org/blink/trunk@185290 bbb929c8-8fbe-4397-9dbb-9b2b20218538