Remove RenderLayerCompositor::m_viewportConstrainedLayers

This member variable and m_viewportConstrainedLayersNeedingUpdate aren't used
by anything.

R=vollick@chromium.org

Review URL: https://codereview.chromium.org/270633003

git-svn-id: svn://svn.chromium.org/blink/trunk@173559 bbb929c8-8fbe-4397-9dbb-9b2b20218538