Remove m_shouldUpdateWhileOffscreen member from FrameView

Remove m_shouldUpdateWhileOffscreen member from FrameView. It is never updated
and it is always true. We should also get rid of the shouldUpdate() method as
it returns true unconditionally.

R=esprehn, adamk

Review URL: https://codereview.chromium.org/138163004

git-svn-id: svn://svn.chromium.org/blink/trunk@165183 bbb929c8-8fbe-4397-9dbb-9b2b20218538