Revert of Remove typedefs.h from webrtc/ root (part 1)

webrtc/rtc_base/annotations.h

-/*
- *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
- *
- *  Use of this source code is governed by a BSD-style license
- *  that can be found in the LICENSE file in the root of the source
- *  tree. An additional intellectual property rights grant can be found
- *  in the file PATENTS.  All contributing project authors may
- *  be found in the AUTHORS file in the root of the source tree.
- */
-
-#ifndef WEBRTC_RTC_BASE_ANNOTATIONS_H_
-#define WEBRTC_RTC_BASE_ANNOTATIONS_H_
-
-// Annotate a function indicating the caller must examine the return value.
-// Use like:
-//   int foo() RTC_WARN_UNUSED_RESULT;
-// To explicitly ignore a result, cast to void.
-// TODO(kwiberg): Remove when we can use [[nodiscard]] from C++17.
-#if defined(__clang__)
-#define RTC_WARN_UNUSED_RESULT __attribute__((__warn_unused_result__))
-#elif defined(__GNUC__)
-// gcc has a __warn_unused_result__ attribute, but you can't quiet it by
-// casting to void, so we don't use it.
-#define RTC_WARN_UNUSED_RESULT
-#else
-#define RTC_WARN_UNUSED_RESULT
-#endif
-
-// Macro to be used for switch-case fallthrough (required for enabling
-// -Wimplicit-fallthrough warning on Clang).
-#if defined(__clang__)
-#define RTC_FALLTHROUGH() [[clang::fallthrough]]
-#else
-#define RTC_FALLTHROUGH() do { } while (0)
-#endif
-
-// Prevent the compiler from warning about an unused variable. For example:
-//   int result = DoSomething();
-//   assert(result == 17);
-//   RTC_UNUSED(result);
-// Note: In most cases it is better to remove the unused variable rather than
-// suppressing the compiler warning.
-#define RTC_UNUSED(x) static_cast<void>(x)
-
-#endif  // WEBRTC_RTC_BASE_ANNOTATIONS_H_