DevTools: Change the color of weak references in heap snapshot

Make it similar to the color of internal refs, not the color of user properties.

R=yurys@chromium.org

Review URL: https://codereview.chromium.org/145273013

git-svn-id: svn://svn.chromium.org/blink/trunk@165745 bbb929c8-8fbe-4397-9dbb-9b2b20218538