Refactoring: toPluginView and toPluginDocument implemented but not used.

Convenience methods toPluginView & toPluginDocument already added but still
using the non-preferred static_cast in usage.

R=tkent

BUG=NONE

TEST=None; No behavior change

Review URL: https://chromiumcodereview.appspot.com/24056004

git-svn-id: svn://svn.chromium.org/blink/trunk@157556 bbb929c8-8fbe-4397-9dbb-9b2b20218538