Delete displayName usages from HeapSnapshotGridNodes.js

That property was used only for native heap snapshots. We don't support them any more.

BUG=None
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/61773011

git-svn-id: svn://svn.chromium.org/blink/trunk@161518 bbb929c8-8fbe-4397-9dbb-9b2b20218538