Define rtc::BufferT, like rtc::Buffer but for any trivial type

And redefine rtc::Buffer as

  using Buffer = BufferT<uint8_t>;

(In the long run, I'd like to remove the type alias and rename the
template to just rtc::Buffer, but that requires all current users of
Buffer to start saying Buffer<uint8_t> instead, and since Buffer is
used in the API, we can't do that in one step.)

The immediate reason for the new template is that we'd like to use
BufferT<int16_t> in the AudioDecoder interface.

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/1929903002
Cr-Commit-Position: refs/heads/master@{#12564}

webrtc/base/buffer.cc

@@ -1,43 +0,0 @@
-/*
- *  Copyright 2015 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.
- */
-
-#include "webrtc/base/buffer.h"
-
-#include <algorithm>
-#include <utility>
-
-namespace rtc {
-
-Buffer::Buffer() : size_(0), capacity_(0), data_(nullptr) {
-  RTC_DCHECK(IsConsistent());
-}
-
-Buffer::Buffer(Buffer&& buf)
-    : size_(buf.size()),
-      capacity_(buf.capacity()),
-      data_(std::move(buf.data_)) {
-  RTC_DCHECK(IsConsistent());
-  buf.OnMovedFrom();
-}
-
-Buffer::Buffer(size_t size) : Buffer(size, size) {
-}
-
-Buffer::Buffer(size_t size, size_t capacity)
-    : size_(size),
-      capacity_(std::max(size, capacity)),
-      data_(new uint8_t[capacity_]) {
-  RTC_DCHECK(IsConsistent());
-}
-
-// Note: The destructor works even if the buffer has been moved from.
-Buffer::~Buffer() = default;
-
-};  // namespace rtc