Make shouldTransformOnTextPainting compilation unit local

No need to have this as a public function of the class. It's only used
in one place, local to the class implementation.

Review URL: https://codereview.chromium.org/292663008

git-svn-id: svn://svn.chromium.org/blink/trunk@174633 bbb929c8-8fbe-4397-9dbb-9b2b20218538