Emit UpdateLayerTree event from InspectorTimelineAgent

This emits a timeline event when layer tree is rebuilt. The layer tree
is built by the LayerTreeAgent.

BUG=

Review URL: https://codereview.chromium.org/196133009

git-svn-id: svn://svn.chromium.org/blink/trunk@169156 bbb929c8-8fbe-4397-9dbb-9b2b20218538