Remove FrameView::m_shouldUpdateWhileOffscreen and logic

This feature was for a WebView API, tracing back 5+ years from
https://trac.webkit.org/changeset/35868

We don't expose the setter and the value is always true so we can just
remove the code.

Review URL: https://codereview.chromium.org/139073002

git-svn-id: svn://svn.chromium.org/blink/trunk@165282 bbb929c8-8fbe-4397-9dbb-9b2b20218538