DevTools: Refactor MemoryStatistics.Counter to represent a single graph.

R=yurys@chromium.org

Review URL: https://codereview.chromium.org/131513023

git-svn-id: svn://svn.chromium.org/blink/trunk@166133 bbb929c8-8fbe-4397-9dbb-9b2b20218538