Add new chrome://proximity-auth WebUI for debugging Smart Lock.

components/proximity_auth/webui/BUILD.gn

Index: components/proximity_auth/webui/BUILD.gn
diff --git a/components/proximity_auth/webui/BUILD.gn b/components/proximity_auth/webui/BUILD.gn
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+# Copyright 2015 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.
+
+# GYP version: components/proximity_auth.gypi:proximity_auth_webui
+static_library("webui") {
+  sources = [
+    "proximity_auth_ui.cc",
+    "proximity_auth_ui.h",
+    "proximity_auth_webui_handler.cc",
+    "proximity_auth_webui_handler.h",
+    "url_constants.cc",
+    "url_constants.h",
+  ]
+
+  deps = [
+    "//base",
+    "//components/proximity_auth",
+    "//components/proximity_auth/cryptauth",
+    "//components/resources",
+    "//content/public/browser",
+  ]
+}