Revert of Remove remains of webrtc/base (patchset #7 id:120001 of https://codereview.webrtc.org/2973183002/ )

Reason for revert:
Breaks lots of downstream projects.

Original issue's description:
> Remove remains of webrtc/base
>
> All downstream code have been updated to the new location.
>
> In PRESUBMIT.py:
> * Remove webrtc/rtc_base from CPP_BLACKLIST
> * Add webrtc/rtc_base to LEGACY_API_DIRS
>
> Fix some duplicated paths in
> webrtc/modules/audio_processing/test/conversational_speech/BUILD.gn
>
> BUG=webrtc:7634
> TBR=kwiberg@webrtc.org
>
> Review-Url: https://codereview.webrtc.org/2973183002
> Cr-Commit-Position: refs/heads/master@{#18948}
> Committed:
9483b49baf

TBR=kwiberg@webrtc.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7634

Review-Url: https://codereview.webrtc.org/2976633002
Cr-Commit-Position: refs/heads/master@{#18949}

webrtc/base/callback.h

@@ -0,0 +1,70 @@
+// This file was GENERATED by command:
+//     pump.py callback.h.pump
+// DO NOT EDIT BY HAND!!!
+
+/*
+ *  Copyright 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.
+ */
+
+// To generate callback.h from callback.h.pump, execute:
+// /home/build/google3/third_party/gtest/scripts/pump.py callback.h.pump
+
+// Callbacks are callable object containers. They can hold a function pointer
+// or a function object and behave like a value type. Internally, data is
+// reference-counted, making copies and pass-by-value inexpensive.
+//
+// Callbacks are typed using template arguments.  The format is:
+//   CallbackN<ReturnType, ParamType1, ..., ParamTypeN>
+// where N is the number of arguments supplied to the callable object.
+// Callbacks are invoked using operator(), just like a function or a function
+// object. Default-constructed callbacks are "empty," and executing an empty
+// callback does nothing. A callback can be made empty by assigning it from
+// a default-constructed callback.
+//
+// Callbacks are similar in purpose to std::function (which isn't available on
+// all platforms we support) and a lightweight alternative to sigslots. Since
+// they effectively hide the type of the object they call, they're useful in
+// breaking dependencies between objects that need to interact with one another.
+// Notably, they can hold the results of Bind(), std::bind*, etc, without
+// needing
+// to know the resulting object type of those calls.
+//
+// Sigslots, on the other hand, provide a fuller feature set, such as multiple
+// subscriptions to a signal, optional thread-safety, and lifetime tracking of
+// slots. When these features are needed, choose sigslots.
+//
+// Example:
+//   int sqr(int x) { return x * x; }
+//   struct AddK {
+//     int k;
+//     int operator()(int x) const { return x + k; }
+//   } add_k = {5};
+//
+//   Callback1<int, int> my_callback;
+//   cout << my_callback.empty() << endl;  // true
+//
+//   my_callback = Callback1<int, int>(&sqr);
+//   cout << my_callback.empty() << endl;  // false
+//   cout << my_callback(3) << endl;  // 9
+//
+//   my_callback = Callback1<int, int>(add_k);
+//   cout << my_callback(10) << endl;  // 15
+//
+//   my_callback = Callback1<int, int>();
+//   cout << my_callback.empty() << endl;  // true
+
+#ifndef WEBRTC_BASE_CALLBACK_H_
+#define WEBRTC_BASE_CALLBACK_H_
+
+
+// This header is deprecated and is just left here temporarily during
+// refactoring. See https://bugs.webrtc.org/7634 for more details.
+#include "webrtc/rtc_base/callback.h"
+
+#endif  // WEBRTC_BASE_CALLBACK_H_