DevTools: Add gradient fill to the timeline memory overview

Also made graph line thinner.

R=yurys@chromium.org

Review URL: https://codereview.chromium.org/93593004

git-svn-id: svn://svn.chromium.org/blink/trunk@164092 bbb929c8-8fbe-4397-9dbb-9b2b20218538