DevTools: Make sure min and max counter values are always defined

Make sure min and max get values even if there's no points with
GPU memory defined.

R=yurys@chromium.org

Review URL: https://codereview.chromium.org/118333002

git-svn-id: svn://svn.chromium.org/blink/trunk@164097 bbb929c8-8fbe-4397-9dbb-9b2b20218538