Replace deprecated base::NonThreadSafe in /cc in favor of SequenceChecker.

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 0a03149c571834e14306e895c484417360fba972..9ab8f6e5618a273a30034c1e248eb3112662ad79 100644
--- a/cc/test/fake_external_begin_frame_source.h
+++ b/cc/test/fake_external_begin_frame_source.h
@@ -9,7 +9,7 @@
 
 #include "base/cancelable_callback.h"
 #include "base/memory/weak_ptr.h"
-#include "base/threading/non_thread_safe.h"
+#include "base/sequence_checker.h"
 #include "cc/output/begin_frame_args.h"
 #include "cc/scheduler/begin_frame_source.h"
 
@@ -19,9 +19,7 @@ class SimpleTestTickClock;
 
 namespace cc {
 
-class FakeExternalBeginFrameSource
-    : public BeginFrameSource,
-      public NON_EXPORTED_BASE(base::NonThreadSafe) {
+class FakeExternalBeginFrameSource : public BeginFrameSource {
  public:
   class Client {
    public:
@@ -63,6 +61,9 @@ class FakeExternalBeginFrameSource
   uint64_t next_begin_frame_number_ = BeginFrameArgs::kStartingFrameNumber;
   std::set<BeginFrameObserver*> observers_;
   base::CancelableCallback<void(const BeginFrameArgs&)> begin_frame_task_;
+
+  SEQUENCE_CHECKER(sequence_checker_);
+
   base::WeakPtrFactory<FakeExternalBeginFrameSource> weak_ptr_factory_;
 };