Refactor part of CompositorImpl::SetVisible() into its own method.

Add method CompositorImpl::CreateLayerTreeHost() and move part of
CompositorImpl::SetVisible() that creates LayerTreeHost into it.

BUG=

Review URL: https://codereview.chromium.org/782723002

Cr-Commit-Position: refs/heads/master@{#307221}
2 files changed