Delete RenderLayerCompositor::m_compositingLayersNeedRebuild

This CL folds m_compositingLayersNeedRebuild into m_updateType, which lets us
remove a bunch of parallel bookkeeping.

BUG=332248

Review URL: https://codereview.chromium.org/320103005

git-svn-id: svn://svn.chromium.org/blink/trunk@175760 bbb929c8-8fbe-4397-9dbb-9b2b20218538