DevTools: Remove stripes from heap snapshot view

This speeds up the view and makes design more neat.

R=pfeldman@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/149173008

git-svn-id: svn://svn.chromium.org/blink/trunk@166130 bbb929c8-8fbe-4397-9dbb-9b2b20218538