CC Animation: Move ElementId from animation to trees.

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

Review-Url: https://codereview.chromium.org/2492133002
Cr-Commit-Position: refs/heads/master@{#432049}

• cc/BUILD.gn[diff]
• cc/animation/animation_events.h[diff]
• cc/animation/animation_player.h[diff]
• cc/animation/element_animations.h[diff]
• cc/blink/web_layer_impl.cc[diff]
• cc/layers/layer.h[diff]
• cc/layers/layer_impl.h[diff]
• cc/test/animation_test_common.h[diff]
• cc/trees/element_id.cc[Renamed from cc/animation/element_id.cc - diff]
• cc/trees/element_id.h[Renamed from cc/animation/element_id.h - diff]
• cc/trees/mutator_host.h[diff]
• cc/trees/mutator_host_client.h[diff]
• cc/trees/property_tree.h[diff]
• cc/trees/scroll_node.cc[diff]
• third_party/WebKit/Source/platform/graphics/CompositorElementId.h[diff]