Remove a bunch of redundant code from CompositedLayerMapping

There are 3 changes here:
1.  Removing some unecessary locals for FloatPoint().
2.  Removing unecessary removeFromParent calls (addChild takes care of that).
3.  Restructuring updateOverflowControlsLayers to use a (way simpler!) helper.

BUG=338896

Review URL: https://codereview.chromium.org/144483005

git-svn-id: svn://svn.chromium.org/blink/trunk@166072 bbb929c8-8fbe-4397-9dbb-9b2b20218538