Remove "mixin" from BeginFrameSource/Observer base classes.

cc/scheduler/scheduler_unittest.cc

Index: cc/scheduler/scheduler_unittest.cc
diff --git a/cc/scheduler/scheduler_unittest.cc b/cc/scheduler/scheduler_unittest.cc
index 3fc5c062dac2c871096d08609064883b14892e76..9e9fe68e205450bafa50dc4e0dc2a919f0120bae 100644
--- a/cc/scheduler/scheduler_unittest.cc
+++ b/cc/scheduler/scheduler_unittest.cc
@@ -228,7 +228,7 @@ class SchedulerClientWithFixedEstimates : public FakeSchedulerClient {
   base::TimeDelta commit_to_activate_duration_;
 };
 
-class FakeExternalBeginFrameSource : public BeginFrameSourceMixIn {
+class FakeExternalBeginFrameSource : public BeginFrameSourceBase {
  public:
   explicit FakeExternalBeginFrameSource(FakeSchedulerClient* client)
       : client_(client) {}