Remove "mixin" from BeginFrameSource/Observer base classes.

content/renderer/gpu/compositor_external_begin_frame_source.h

Index: content/renderer/gpu/compositor_external_begin_frame_source.h
diff --git a/content/renderer/gpu/compositor_external_begin_frame_source.h b/content/renderer/gpu/compositor_external_begin_frame_source.h
index 839f6cffa3babb7511d97dbbde5de6c4cf72cd8f..92f9184dc5d329222c69c95bc23fa962126f5849 100644
--- a/content/renderer/gpu/compositor_external_begin_frame_source.h
+++ b/content/renderer/gpu/compositor_external_begin_frame_source.h
@@ -21,7 +21,7 @@ namespace content {
 // This class can be created only on the main thread, but then becomes pinned
 // to a fixed thread where cc::Scheduler is running.
 class CompositorExternalBeginFrameSource
-    : public cc::BeginFrameSourceMixIn,
+    : public cc::BeginFrameSourceBase,
       public NON_EXPORTED_BASE(base::NonThreadSafe) {
  public:
   explicit CompositorExternalBeginFrameSource(
@@ -30,7 +30,7 @@ class CompositorExternalBeginFrameSource
       int routing_id);
   ~CompositorExternalBeginFrameSource() override;
 
-  // cc::BeginFrameSourceMixIn implementation.
+  // cc::BeginFrameSourceBase implementation.
   void OnNeedsBeginFramesChange(bool needs_begin_frames) override;
   void SetClientReady() override;