DevTools: hide page overlay layers in LayerTreeAgent

Filter out layers that are used by page overlays when reporting layer tree to the front-end.

Drive-by: move layer tests to inspector/layers.

BUG=

Review URL: https://codereview.chromium.org/173763002

git-svn-id: svn://svn.chromium.org/blink/trunk@167677 bbb929c8-8fbe-4397-9dbb-9b2b20218538