Land layer count based benchmark tests.

Picture layers always push properties on Activation.

PictureLayerTilingSet always compute priority rectangle changes.

PictureLayerTilingSet visible_rect_history updates are a hotspot.

Copying the LayersThatShouldPushProperties set in TreeSynchronizer::PushLayerProperties is a hotspot.

Composited scrollbars are re-painted and committed for composited scrollers.

Consider options to early out layer iteration in RasterTilePriorityQueueAll & RasterTilePriorityQueueRequired.

Skip TileManager::IsReadyToActivate and IsReadyToDraw computation if not needed.

Scroll offset changes for non-root scrollable paint layers trigger graphics layer updates.

Need a layer performance test with viewport constrained layers.