DevTools: Fix typo causing exception on heap profiles with no target.

TBR=yurys@chromium.org,loislo@chromium.org

Review URL: https://codereview.chromium.org/547413003

git-svn-id: svn://svn.chromium.org/blink/trunk@181568 bbb929c8-8fbe-4397-9dbb-9b2b20218538