[CrOS MultiDevice] Create a skeleton MultiDeviceSetup service.

See go/unified-better-together-cros for an outline of this service.

This CL does the following:
(1) Creates a Mojo interface for the service, adding only a single
    function for now.
(2) Creates the MultiDeviceSetupService and MultiDeviceSetupImpl classes
    which implement this interface, as well as a test for the service.
(3) Registers the service in the browser process via ProfileImpl.
(4) Generates JavaScript files for the interface; because these files
    are the first resources generated in //chromeos, this CL also adds a
    new chromeos_resources.grd.
(5) Adds the new "multidevice_setup" capability to the content_browser
    manifest file so that it can be used by content_renderer.
(6) Adds a new button on the chrome://proximity-auth debug WebUI page
    which makes a Mojo call to the service.

Change-Id: I0b74373b78a1c92fe01d789028342707321c2134
Reviewed-on: https://chromium-review.googlesource.com/954010
Reviewed-by: Steven Bennetts <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: agrieve <[email protected]>
Reviewed-by: anthonyvd <[email protected]>
Reviewed-by: Ken Rockot <[email protected]>
Commit-Queue: Kyle Horimoto <[email protected]>
Cr-Commit-Position: refs/heads/master@{#543894}
diff --git a/chromeos/resources/BUILD.gn b/chromeos/resources/BUILD.gn
new file mode 100644
index 0000000..4b64e4e5
--- /dev/null
+++ b/chromeos/resources/BUILD.gn
@@ -0,0 +1,28 @@
+# Copyright 2018 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.
+
+import("//tools/grit/grit_rule.gni")
+
+assert(is_chromeos, "Non-ChromeOS builds cannot depend on //chromeos")
+
+grit("resources") {
+  source = "chromeos_resources.grd"
+
+  source_is_generated = true
+
+  outputs = [
+    "grit/chromeos_resources.h",
+    "chromeos_resources.pak",
+  ]
+  output_dir = "$root_gen_dir/chromeos"
+
+  grit_flags = [
+    "-E",
+    "mojom_root=" + rebase_path(root_gen_dir, root_build_dir),
+  ]
+
+  deps = [
+    "//chromeos/services/multidevice_setup/public/mojom:mojom_js",
+  ]
+}