Add toPluginDocument() convenience method that takes a reference in argument

Add toPluginDocument() convenience method that takes a reference in argument
to avoid having to convert references into pointers at call sites.

R=tkent
BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23549009

git-svn-id: svn://svn.chromium.org/blink/trunk@157075 bbb929c8-8fbe-4397-9dbb-9b2b20218538