Implement proper synchronization between HW video decode IPC and CommandBuffer.

gpu/command_buffer/common/command_buffer.cc

+// Copyright (c) 2011 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.
+
+#include "../common/command_buffer.h"
+#include "../common/logging.h"
+
+namespace gpu {
+
+ReadWriteTokens::ReadWriteTokens()
+    : last_token_read(-1), last_token_written(-1) {
+}
+
+ReadWriteTokens::ReadWriteTokens(int32 read, int32 written)
+    : last_token_read(read), last_token_written(written) {
+}
+
+bool ReadWriteTokens::InRange(int32 token) const {
+  int32 min = last_token_read;
+  int32 max = last_token_written;
+  GPU_DCHECK_GE(min, 0);
+  GPU_DCHECK_GE(max, 0);
+  if (min <= max) {
+    // token should be in [min .. max)
+    return (token >= min) && (token < max);
+  }
+  // token should be in [0 .. max) or [min .. wrap token)
+  return (token >= min) || (token < max);
+}
+
+}  // namespace gpu