DevTools: Show the last valid value for GPU memory counter.

Currently the graph drops to zero at the chart righthand side.

R=yurys@chromium.org

Review URL: https://codereview.chromium.org/117243002

git-svn-id: svn://svn.chromium.org/blink/trunk@164016 bbb929c8-8fbe-4397-9dbb-9b2b20218538