Make SimpleStringBuilder into a non-template

So that future CLs can de-inline its methods.

We do this by asking the caller to allocate the buffer instead of
having it as a data member.

Bug: webrtc:8982
Change-Id: I246b0973e54510fdd880c3b6875336c31334d008
Reviewed-on: https://webrtc-review.googlesource.com/60000
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22355}

rtc_base/strings/string_builder.cc

@@ -0,0 +1,21 @@
+/*
+ *  Copyright 2018 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 "rtc_base/strings/string_builder.h"
+
+namespace rtc {
+
+SimpleStringBuilder::SimpleStringBuilder(rtc::ArrayView<char> buffer)
+    : buffer_(buffer) {
+  buffer_[0] = '\0';
+  RTC_DCHECK(IsConsistent());
+}
+
+}  // namespace rtc