Remove TimelinePresentationModel.Record.childHasWarning

The functionlity is already supported by presentation record.

BUG=None
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/306123009

git-svn-id: svn://svn.chromium.org/blink/trunk@175265 bbb929c8-8fbe-4397-9dbb-9b2b20218538