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diff --git a/chromium/content/common/state_transitions.h b/chromium/content/common/state_transitions.h
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+++ b/chromium/content/common/state_transitions.h
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+// Copyright 2020 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 CONTENT_COMMON_STATE_TRANSITIONS_H_
+#define CONTENT_COMMON_STATE_TRANSITIONS_H_
+
+#include <vector>
+
+#include "base/check_op.h"
+#include "base/no_destructor.h"
+#include "base/stl_util.h"
+
+namespace content {
+
+// This class represents a set of state transitions where each state is a value
+// that supports copy, << and == (e.g. an enum element). It's intended to be
+// used in DCHECK-enabled builds to check that only valid transitions occur. Its
+// implementation favours convenience and simplicity over performance. To use it
+// follow this example:
+
+// In foo.h
+// ---------
+// enum class State {
+//   kState1,
+//   kState2,
+//   kState3,
+// };
+//
+// // This may require exporting the symbol (e.g. CONTENT_EXPORT) if it will be
+// // used by any other components: one common way this can happen is if the
+// // enum is logged in tests (e.g. via gtest's EXPECT_* macros).
+// std::ostream& operator<<(std::ostream& o, const State& s);
+// ---------
+//
+// In foo.cc
+// ---------
+// #include "base/no_destructor.h"
+// #include "content/common/state_transitions.h"
+//
+// std::ostream& operator<<(std::ostream& o, const State& s) {
+//   return o << static_cast<int>(s);
+// }
+//
+// void DCheckStateTransition(State old_state, State new_state) {
+// #if DCHECK_IS_ON()
+//   static const base::NoDestructor<StateTransitions<State>> transitions(
+//       StateTransitions<State>({
+//           {kState1, {kState2, kState3}},
+//           {kState2, {kState3}},
+//           {kState3, {}},
+//       }));
+//   DCHECK_STATE_TRANSITION(transitions, old_state, new_state);
+// #endif  // DCHECK_IS_ON()
+// }
+// ---------
+
+template <typename State>
+struct StateTransitions {
+ public:
+  // Represents a state and all of the states that are valid transitions from
+  // it.
+  struct StateTransition {
+    StateTransition(State source, std::vector<State> destinations)
+        : source(std::move(source)), destinations(std::move(destinations)) {}
+
+    const State source;
+    const std::vector<State> destinations;
+  };
+
+  explicit StateTransitions(std::vector<StateTransition> state_transitions)
+      : state_transitions(std::move(state_transitions)) {}
+
+  // Returns a list of states that are valid to transition to from |source|.
+  const std::vector<State>& GetValidTransitions(const State& source) const {
+    for (const StateTransition& state_transition : state_transitions) {
+      if (state_transition.source == source)
+        return state_transition.destinations;
+    }
+    static const base::NoDestructor<std::vector<State>> no_transitions;
+    return *no_transitions;
+  }
+
+  // Tests whether transitioning from |source| to |destination| is valid.
+  bool IsTransitionValid(const State& source, const State& destination) const {
+    return base::Contains(GetValidTransitions(source), destination);
+  }
+
+  const std::vector<StateTransition> state_transitions;
+};
+
+// DCHECK if transitioning from |old_state| to |new_state| is not valid
+// according to |transitions|.
+#define DCHECK_STATE_TRANSITION(transitions, old_state, new_state)   \
+  DCHECK((transitions)->IsTransitionValid((old_state), (new_state))) \
+      << "Invalid transition: " << old_state << " -> " << new_state
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_STATE_TRANSITIONS_H_