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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