Remove "mixin" from BeginFrameSource/Observer base classes.

cc/test/fake_external_begin_frame_source.h

Index: cc/test/fake_external_begin_frame_source.h
diff --git a/cc/test/fake_external_begin_frame_source.h b/cc/test/fake_external_begin_frame_source.h
index d5b14e7556d3fdb1dd4d7c126c96250019465119..31e1f452e881b7cb4bf359e1f0b527607e980fce 100644
--- a/cc/test/fake_external_begin_frame_source.h
+++ b/cc/test/fake_external_begin_frame_source.h
@@ -11,7 +11,7 @@
 namespace cc {
 
 class FakeExternalBeginFrameSource
-    : public BeginFrameSourceMixIn,
+    : public BeginFrameSourceBase,
       public NON_EXPORTED_BASE(base::NonThreadSafe) {
  public:
   explicit FakeExternalBeginFrameSource(double refresh_rate);
@@ -22,7 +22,7 @@ class FakeExternalBeginFrameSource
   // BeginFrameSource implementation.
   void SetClientReady() override;
 
-  // BeginFrameSourceMixIn overrides.
+  // BeginFrameSourceBase overrides.
   void OnNeedsBeginFramesChange(bool needs_begin_frames) override;
 
   void TestOnBeginFrame();