Removing remaining spurrious NULL checks on m_frame in FrameView

Amendment to CL: https://codereview.chromium.org/23483024

BUG=

Review URL: https://chromiumcodereview.appspot.com/23551010

git-svn-id: svn://svn.chromium.org/blink/trunk@157387 bbb929c8-8fbe-4397-9dbb-9b2b20218538