Moved RenderGeometryMapStep into its own file.

R=abarth@chromium.org, esprehn@chromium.org, vollick@chromium.org

Review URL: https://codereview.chromium.org/408363002

git-svn-id: svn://svn.chromium.org/blink/trunk@178710 bbb929c8-8fbe-4397-9dbb-9b2b20218538