Make RenderGeometryMap::dumpSteps const.

RenderGeometryMap::dumpSteps doesn't have any side effects other then writing
information to stderr. Marking it const so we can use it from other const
methods.

NOTRY=true

Review URL: https://codereview.chromium.org/301243006

git-svn-id: svn://svn.chromium.org/blink/trunk@175145 bbb929c8-8fbe-4397-9dbb-9b2b20218538