Timeline: fix expansion arrow appearing on rows with no visible children

We used to check TimelinePresentationRecord.expandable(), but it was accidrently replaced
with hasPresentationChildren(), which is not the same now, since we keep hidden records
in the presentation model.

R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/201823003

git-svn-id: svn://svn.chromium.org/blink/trunk@169620 bbb929c8-8fbe-4397-9dbb-9b2b20218538