Move rw_lock_wrapper.h to rtc_base/synchronization/

This moves it from an API directory (system_wrappers/include/) to a
non-API directory, which is exactly what we want for utilities like
this.

BUG=webrtc:8445
NOPRESUBMIT=true

Change-Id: Ie2879aca5fc1667e4222499d2a8fc2bba9ae2425
Reviewed-on: https://webrtc-review.googlesource.com/21328
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22587}

rtc_base/synchronization/BUILD.gn

@@ -0,0 +1,38 @@
+# Copyright (c) 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.
+
+import("../../webrtc.gni")
+if (is_android) {
+  import("//build/config/android/config.gni")
+  import("//build/config/android/rules.gni")
+}
+
+rtc_source_set("rw_lock_wrapper") {
+  public = [
+    "rw_lock_wrapper.h",
+  ]
+  sources = [
+    "rw_lock_wrapper.cc",
+  ]
+  deps = [
+    "..:macromagic",
+    "../..:typedefs",
+  ]
+  if (is_win) {
+    sources += [
+      "rw_lock_win.cc",
+      "rw_lock_win.h",
+    ]
+    deps += [ "..:logging" ]
+  } else {
+    sources += [
+      "rw_lock_posix.cc",
+      "rw_lock_posix.h",
+    ]
+  }
+}