Centralize FrameSinkId allocation in one place

cc/surfaces/frame_sink_id_allocator.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CC_SURFACES_FRAME_SINK_ID_ALLOCATOR_H_
+#define CC_SURFACES_FRAME_SINK_ID_ALLOCATOR_H_
+
+#include "cc/surfaces/frame_sink_id.h"
+
+namespace cc {
+
+// This class generates FrameSinkId with a fixed client_id and an
+// incrementally-increasing sink_id.
+class FrameSinkIdAllocator {
+ public:
+  constexpr explicit FrameSinkIdAllocator(uint32_t client_id)
+      : client_id_(client_id), next_sink_id_(1u) {}
+
+  FrameSinkId NextFrameSinkId() {
+    return FrameSinkId(client_id_, next_sink_id_++);
+  }
+
+ private:
+  const uint32_t client_id_;
+  uint32_t next_sink_id_;
+
+  DISALLOW_COPY_AND_ASSIGN(FrameSinkIdAllocator);
+};
+
+}  // namespace cc
+
+#endif  // CC_SURFACES_FRAME_SINK_ID_ALLOCATOR_H_