Remove "mixin" from BeginFrameSource/Observer base classes.

cc/surfaces/display_scheduler.cc

Index: cc/surfaces/display_scheduler.cc
diff --git a/cc/surfaces/display_scheduler.cc b/cc/surfaces/display_scheduler.cc
index ea01318a0766ab1cc54773deeb7b098818e6e810..71babc035d00fa5f9c75bc045444fb65b1506f1a 100644
--- a/cc/surfaces/display_scheduler.cc
+++ b/cc/surfaces/display_scheduler.cc
@@ -122,7 +122,7 @@ void DisplayScheduler::DrawAndSwap() {
   root_surface_damaged_ = false;
 }
 
-bool DisplayScheduler::OnBeginFrameMixInDelegate(const BeginFrameArgs& args) {
+bool DisplayScheduler::OnBeginFrameDerivedImpl(const BeginFrameArgs& args) {
   base::TimeTicks now = base::TimeTicks::Now();
   TRACE_EVENT2("cc", "DisplayScheduler::BeginFrame", "args", args.AsValue(),
                "now", now);