DevTools: REGRESSION fix timeline JS memory overview after r172735 and r172617

TBR=loislo
BUG=361045

Review URL: https://codereview.chromium.org/258893008

git-svn-id: svn://svn.chromium.org/blink/trunk@172743 bbb929c8-8fbe-4397-9dbb-9b2b20218538