Revert "CryptAuth: Remove unused ConnectionFinder."

This reverts commit 6c1f4453.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 607302 as the
culprit for failures in the build cycles as shown on:
https://findit-for-me.appspot.com/waterfall/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyRAsSDVdmU3VzcGVjdGVkQ0wiMWNocm9taXVtLzZjMWY0NDUzYjQ1NjdlYmFlNjljYTlmNDJmZDY2MDJlMDZkMWIxZGMM

Sample Failed Build: https://ci.chromium.org/buildbot/chromium.chromiumos/Linux%20ChromiumOS%20Full/109736

Sample Failed Step: compile

Original change's description:
> CryptAuth: Remove unused ConnectionFinder.
> 
> Bug: 899324
> Test: none
> Change-Id: I30f07136ac7c3ed3ac49b678e5f45d250fcbe65f
> Reviewed-on: https://chromium-review.googlesource.com/c/1331573
> Reviewed-by: Jeremy Klein <jlklein@chromium.org>
> Commit-Queue: James Hawkins <jhawkins@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#607302}

Change-Id: I0c821dda827a0daf2a144b60718e370c459abd6f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 899324
Reviewed-on: https://chromium-review.googlesource.com/c/1331948
Cr-Commit-Position: refs/heads/master@{#607310}

components/cryptauth/connection_finder.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_CRYPTAUTH_CONNECTION_FINDER_H_
+#define COMPONENTS_CRYPTAUTH_CONNECTION_FINDER_H_
+
+#include <memory>
+
+#include "base/callback_forward.h"
+#include "base/macros.h"
+#include "components/cryptauth/connection.h"
+
+namespace cryptauth {
+
+// Interface for finding a connection to a remote device.
+class ConnectionFinder {
+ public:
+  virtual ~ConnectionFinder() {}
+
+  // Attempts to find a connection to a remote device. The finder will try to
+  // find the connection indefinitely until the finder is destroyed. Calls
+  // |connection_callback| with the open connection once the remote device is
+  // connected.
+  // TODO(isherman): Can this just be done as part of the constructor?
+  typedef base::Callback<void(std::unique_ptr<Connection> connection)>
+      ConnectionCallback;
+  virtual void Find(const ConnectionCallback& connection_callback) = 0;
+};
+
+}  // namespace cryptauth
+
+#endif  // COMPONENTS_CRYPTAUTH_CONNECTION_FINDER_H_