Moving compile_assert_c.h to webrtc/base

This header was not tracked by a GN target and in the discussion on
https://bugs.chromium.org/p/webrtc/issues/detail?id=7617 we decided
to also move it under webrtc/base.

I checked in chromium code search and it seems safe to move it
without creating a stub header in webrtc/system_wrappers.

BUG=webrtc:7617

Review-Url: https://codereview.webrtc.org/2882673002
Cr-Commit-Position: refs/heads/master@{#18151}

webrtc/base/compile_assert_c.h

@@ -0,0 +1,21 @@
+/*
+ *  Copyright (c) 2012 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_BASE_COMPILE_ASSERT_C_H_
+#define WEBRTC_BASE_COMPILE_ASSERT_C_H_
+
+// Use this macro to verify at compile time that certain restrictions are met.
+// The argument is the boolean expression to evaluate.
+// Example:
+//   RTC_COMPILE_ASSERT(sizeof(foo) < 128);
+// Note: In C++, use static_assert instead!
+#define RTC_COMPILE_ASSERT(expression) switch (0) {case 0: case expression:;}
+
+#endif  // WEBRTC_BASE_COMPILE_ASSERT_C_H_