Missing Markers filter toolbar button after restoring timegraph from persisted state

[PatrickTasse]

If a trace is opened with a timegraph that contains Marker annotations, the Markers filter toolbar button appears on the trace viewer toolbar. If the browser tab is closed then reopened, the timegraph is restored from the persisted state but the Markers filter toolbar button does not reappear. The timegraph must be closed and reopened to fix the toolbar.