Extract paint profiler from LayerTreeModel

It is logically unrelated to other functionality of LayerTreeModel and extracting
makes it easier to reuse in panels other than Layers. Note we also drop unused
reference from snapshot to layer and rename LayerSnapshot to PaintProfilerSnapshot.

Review URL: https://codereview.chromium.org/120043003

git-svn-id: svn://svn.chromium.org/blink/trunk@164282 bbb929c8-8fbe-4397-9dbb-9b2b20218538