Assume FrameView::m_frame is never NULL

FrameView::clearFrame() is unused and we never pass a null frame pointer to FrameView::create(),
so FrameView::m_frame should never be NULL. This removes FrameView::clearFrame() and
removes m_frame check from conditional expressions.

BUG=
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/23483024

git-svn-id: svn://svn.chromium.org/blink/trunk@157068 bbb929c8-8fbe-4397-9dbb-9b2b20218538