Revert "Removing dead ServiceManager constants"

This reverts commit 66630f37.

Reason for revert: Breaks compile on linux-chromeos-rel.

BUG=996442

Original change's description:
> Removing dead ServiceManager constants
> 
> Since ServiceManager went away, we're clearing up some leftover
> CupsProxyService constants. Trivial change.
> 
> Bug: chromium:945409
> Test: confirmed service behavior unchanged
> Change-Id: I45ffdeb3b0ad1990798a4a4d5115e791a6eefbfe
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1764430
> Auto-Submit: Luum Habtemariam <luum@chromium.org>
> Reviewed-by: Ken Rockot <rockot@google.com>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Commit-Queue: Daniel Cheng <dcheng@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#689164}

TBR=dcheng@chromium.org,rockot@google.com,skau@chromium.org,luum@chromium.org

Change-Id: Ic9914332352bc21a15ce96e14f192eefdbed499e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:945409
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1764333Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Auto-Submit: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689201}

chrome/services/cups_proxy/public/mojom/BUILD.gn

@@ -7,6 +7,7 @@ import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("mojom") {
   sources = [
+    "constants.mojom",
     "proxy.mojom",
   ]
 

chrome/services/cups_proxy/public/mojom/constants.mojom

+// Copyright 2019 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.
+
+module chromeos.printing.mojom;
+
+// Service id.
+const string kCupsProxyServiceName = "cups_proxy";
+
+// StartCupsProxyService capability id.
+const string kStartCupsProxyServiceCapability = "start_cups_proxy_service";