Rename CompositingUpdateOnCompositedScroll to CompositingUpdateAfterGeometryChange

What this update type actually does is update the geometry of all the
composited layer mappings that have dirty geometry. Rather than naming the
value for its caller, we should name the value for its effect.

R=vollick@chromium.org

Review URL: https://codereview.chromium.org/334973002

git-svn-id: svn://svn.chromium.org/blink/trunk@176127 bbb929c8-8fbe-4397-9dbb-9b2b20218538