Remove "mixin" from BeginFrameSource/Observer base classes.

cc/scheduler/scheduler.cc

Index: cc/scheduler/scheduler.cc
diff --git a/cc/scheduler/scheduler.cc b/cc/scheduler/scheduler.cc
index 0d90c68214f15a822f66419d899885b7a4e84e56..79712aeee078ba1e7d1156175c7b9c3901ad07a8 100644
--- a/cc/scheduler/scheduler.cc
+++ b/cc/scheduler/scheduler.cc
@@ -343,7 +343,7 @@ void Scheduler::SetupPollingMechanisms() {
 // making a frame. Usually this means that user input for the frame is complete.
 // If the scheduler is busy, we queue the BeginFrame to be handled later as
 // a BeginRetroFrame.
-bool Scheduler::OnBeginFrameMixInDelegate(const BeginFrameArgs& args) {
+bool Scheduler::OnBeginFrameDerivedImpl(const BeginFrameArgs& args) {
   TRACE_EVENT1("cc,benchmark", "Scheduler::BeginFrame", "args", args.AsValue());
 
   // Trace this begin frame time through the Chrome stack