Animations: Implement runtime flag for enabling compositor animation timelines.

cc/base/switches.cc

Index: cc/base/switches.cc
diff --git a/cc/base/switches.cc b/cc/base/switches.cc
index 504e787cd66e73a7a69ba0d8a62ba9bd7cfb39d8..93a9b650ef85a75ac047a752b305123087d7a73c 100644
--- a/cc/base/switches.cc
+++ b/cc/base/switches.cc
@@ -63,6 +63,10 @@ const char kDisablePinchVirtualViewport[] = "disable-pinch-virtual-viewport";
 const char kEnablePropertyTreeVerification[] =
     "enable-property-tree-verification";
 
+// Enables new cc/animation system (Project Heaviside). crbug.com/394772
+const char kEnableCompositorAnimationTimelines[] =
+    "enable-compositor-animation-timelines";
+
 // Disable partial swap which is needed for some OpenGL drivers / emulators.
 const char kUIDisablePartialSwap[] = "ui-disable-partial-swap";