CC Animation: Move LayerTreeMutator from cc/animation to cc/trees.

CC shnoudn't inlude headers from cc/animation.

BUG=575043
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2492743002
Cr-Commit-Position: refs/heads/master@{#431393}

• cc/BUILD.gn[diff]
• cc/test/fake_proxy.cc[diff]
• cc/trees/layer_tree_host_impl.h[diff]
• cc/trees/layer_tree_mutator.h[Renamed from cc/animation/layer_tree_mutator.h - diff]
• cc/trees/threaded_channel.cc[diff]
• content/renderer/gpu/render_widget_compositor.cc[diff]
• third_party/WebKit/public/platform/WebCompositorMutatorClient.h[diff]