Fix a bug that sequencial focus navigation is stuck before a plugin elment in an IFRAME.

This is a regression by [1].
Plugin elements need to declare they have custom focus logic in order to make
them focusable while they have ShadowRoots.

[1] http://src.chromium.org/viewvc/blink?revision=163140&view=revision

BUG=340968
TEST=automated

Review URL: https://codereview.chromium.org/148063005

git-svn-id: svn://svn.chromium.org/blink/trunk@166597 bbb929c8-8fbe-4397-9dbb-9b2b20218538