Remove isPluginElement hack in setFocusedNode.
Remove isPluginElement hack in setFocusedNode.

This hack is no longer necessary because [1] made isFocusable for plugin
elements work correctly.

[1] http://src.chromium.org/viewvc/blink?revision=147767&view=revision

TEST=none; no behavior changes.
BUG=none

Review URL: https://chromiumcodereview.appspot.com/14394002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@148800 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs