Animations: Implement runtime flag for enabling compositor animation timelines.

cc/trees/layer_tree_settings.cc

Index: cc/trees/layer_tree_settings.cc
diff --git a/cc/trees/layer_tree_settings.cc b/cc/trees/layer_tree_settings.cc
index 9d605aea8bdbdb23534328a94bf219fc32647963..0fa867601db20358942b41bfa5c63f4bcbd51a72 100644
--- a/cc/trees/layer_tree_settings.cc
+++ b/cc/trees/layer_tree_settings.cc
@@ -72,7 +72,8 @@ LayerTreeSettings::LayerTreeSettings()
       use_occlusion_for_tile_prioritization(false),
       record_full_layer(false),
       use_display_lists(false),
-      verify_property_trees(false) {
+      verify_property_trees(false),
+      use_compositor_animation_timelines(false) {
 }
 
 LayerTreeSettings::~LayerTreeSettings() {}