Animations: Implement runtime flag for enabling compositor animation timelines.

ui/compositor/compositor_switches.cc

Index: ui/compositor/compositor_switches.cc
diff --git a/ui/compositor/compositor_switches.cc b/ui/compositor/compositor_switches.cc
index afbaa580c4a3251cbe7ff9f2ac07961bc1fe0a88..c5f10f4460c62bd9a5dba7110f4a928e09958f8c 100644
--- a/ui/compositor/compositor_switches.cc
+++ b/ui/compositor/compositor_switches.cc
@@ -17,6 +17,9 @@ const char kEnablePixelOutputInTests[] = "enable-pixel-output-in-tests";
 
 const char kUIDisableThreadedCompositing[] = "ui-disable-threaded-compositing";
 
+const char kUIEnableCompositorAnimationTimelines[] =
+    "ui-enable-compositor-animation-timelines";
+
 const char kUIEnableImplSidePainting[] = "ui-enable-impl-side-painting";
 
 const char kUIEnableZeroCopy[] = "ui-enable-zero-copy";