CC Animation: Move MutableProperty enum from animation to trees.

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

Review-Url: https://codereview.chromium.org/2499503002
Cr-Commit-Position: refs/heads/master@{#431512}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 13b00ff..b5d6d7e 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -535,6 +535,7 @@
     "trees/layer_tree_mutator.h",
     "trees/layer_tree_settings.cc",
     "trees/layer_tree_settings.h",
+    "trees/mutable_properties.h",
     "trees/mutator_host.h",
     "trees/mutator_host_client.h",
     "trees/occlusion.cc",