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}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 7b541f1..a5a859e 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -24,8 +24,6 @@
     "animation/animation_timeline.h",
     "animation/element_animations.cc",
     "animation/element_animations.h",
-    "animation/element_id.cc",
-    "animation/element_id.h",
     "animation/keyframed_animation_curve.cc",
     "animation/keyframed_animation_curve.h",
     "animation/property_animation_state.cc",
@@ -510,6 +508,8 @@
     "trees/draw_property_utils.h",
     "trees/effect_node.cc",
     "trees/effect_node.h",
+    "trees/element_id.cc",
+    "trees/element_id.h",
     "trees/latency_info_swap_promise_monitor.cc",
     "trees/latency_info_swap_promise_monitor.h",
     "trees/layer_tree.cc",