Add ThreadLogMessages and its test

util/thread/thread_log_messages.h

Index: util/thread/thread_log_messages.h
diff --git a/util/thread/thread_log_messages.h b/util/thread/thread_log_messages.h
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+// Copyright 2015 The Crashpad Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef CRASHPAD_UTIL_THREAD_THREAD_LOG_MESSAGES_H_
+#define CRASHPAD_UTIL_THREAD_THREAD_LOG_MESSAGES_H_
+
+#include <string>
+#include <vector>
+
+#include "base/basictypes.h"
+
+namespace crashpad {
+
+//! \brief Captures log messages produced on the current thread during an
+//!     object’s lifetime.
+//!
+//! At most one object of this class type may exist on a single thread at a
+//! time. When using this class, no other part of the program may call
+//! `logging::SetLogMessageHandler()` at any time.
+class ThreadLogMessages {
+ public:
+  ThreadLogMessages();
+  ~ThreadLogMessages();
+
+  //! \return The log messages collected on the thread that this object was
+  //!     created on since the time it was created.
+  const std::vector<std::string>& log_messages() const { return log_messages_; }
+
+ private:
+  std::vector<std::string> log_messages_;
+
+  DISALLOW_COPY_AND_ASSIGN(ThreadLogMessages);
+};
+
+}  // namespace crashpad
+
+#endif  // CRASHPAD_UTIL_THREAD_THREAD_LOG_MESSAGES_H_