Centralize FrameSinkId allocation in one place

cc/surfaces/frame_sink_id_allocator.h

Index: cc/surfaces/frame_sink_id_allocator.h
diff --git a/cc/surfaces/frame_sink_id_allocator.h b/cc/surfaces/frame_sink_id_allocator.h
new file mode 100644
index 0000000000000000000000000000000000000000..9ac1c3946479f7ab7025e71d078a0c8f1330ac75
--- /dev/null
+++ b/cc/surfaces/frame_sink_id_allocator.h
@@ -0,0 +1,32 @@
+// 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 FrameSinkIdAllocator(uint32_t client_id)

[Fady Samuel, 2017/02/17 16:00:30]

Another nit, just noticed. This needs explicit.

[danakj, 2017/02/21 22:56:56]

Presubmit should warn about this I thought? Does the constexpr mess it up?

[xlai (Olivia), 2017/02/22 16:24:07]

Done, added explicit. I don't think constexpr and explicit would mess up; they
indicate that these constructor is both in-line and doesn't allow copy
initialization. A lot of ui/gfx/geometry classes use this combination of
keywords.

+      : 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_