Add empty header to simplify Chromium migration.

In order to land http://crrev.com/c/2593640 and make Chromium
backwards compatible with [1], a new empty header needs to be created.

[1] - https://webrtc-review.googlesource.com/c/src/+/196903

Bug: webrtc:9987
Change-Id: I48c4c47b56f9334ce329826c584c817919d59dad
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/197941
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32849}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 32e873d..3448504 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -669,6 +669,7 @@
   # TODO(bugs.webrtc.org/9987): This build target will soon contain
   # async_resolver source files (see
   # https://webrtc-review.googlesource.com/c/src/+/196903).
+  sources = [ "async_resolver.h" ]
 }
 
 rtc_source_set("net_helpers") {
diff --git a/rtc_base/async_resolver.h b/rtc_base/async_resolver.h
new file mode 100644
index 0000000..3c3ad82
--- /dev/null
+++ b/rtc_base/async_resolver.h
@@ -0,0 +1,17 @@
+/*
+ *  Copyright 2008 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 RTC_BASE_ASYNC_RESOLVER_H_
+#define RTC_BASE_ASYNC_RESOLVER_H_
+
+// Placeholder header for the refactoring in:
+// https://webrtc-review.googlesource.com/c/src/+/196903
+
+#endif  // RTC_BASE_ASYNC_RESOLVER_H_