[cc] Calculate the correct latest_confirmed_sequence_number in cc::Scheduler.

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 d02fe0bc7fb11b0b30fe716b5c2f88962aaba4ff..af0c0a99274e671ce0259a012c5447b9b1719f74 100644
--- a/cc/test/fake_external_begin_frame_source.h
+++ b/cc/test/fake_external_begin_frame_source.h
@@ -13,6 +13,10 @@
 #include "cc/output/begin_frame_args.h"
 #include "cc/scheduler/begin_frame_source.h"
 
+namespace base {
+class SimpleTestTickClock;
+}  // namespace base
+
 namespace cc {
 
 class FakeExternalBeginFrameSource
@@ -36,13 +40,23 @@ class FakeExternalBeginFrameSource
   void AddObserver(BeginFrameObserver* obs) override;
   void RemoveObserver(BeginFrameObserver* obs) override;
   void DidFinishFrame(BeginFrameObserver* obs,
-                      const BeginFrameAck& ack) override {}
+                      const BeginFrameAck& ack) override;
   bool IsThrottled() const override;
 
+  BeginFrameArgs CreateBeginFrameArgs(
+      BeginFrameArgs::CreationLocation location);
+  BeginFrameArgs CreateBeginFrameArgs(BeginFrameArgs::CreationLocation location,
+                                      base::SimpleTestTickClock* now_src);
+  uint64_t next_begin_frame_number() const { return next_begin_frame_number_; }
+
   void TestOnBeginFrame(const BeginFrameArgs& args);
 
   size_t num_observers() const { return observers_.size(); }
 
+  const BeginFrameAck& LastAckForObserver(BeginFrameObserver* obs) {
+    return last_acks_[obs];
+  }
+
  private:
   void PostTestOnBeginFrame();
 
@@ -54,6 +68,7 @@ class FakeExternalBeginFrameSource
   uint64_t next_begin_frame_number_ = BeginFrameArgs::kStartingFrameNumber;
   std::set<BeginFrameObserver*> observers_;
   base::CancelableCallback<void(const BeginFrameArgs&)> begin_frame_task_;
+  std::unordered_map<BeginFrameObserver*, BeginFrameAck> last_acks_;
   base::WeakPtrFactory<FakeExternalBeginFrameSource> weak_ptr_factory_;
 };