Fix build with enable_print_preview = false

Originally broken by https://chromium-review.googlesource.com/c/chromium/src/+/2422820
due to an unconditional dependency on //components/pwg_encoder which asserts
enable_print_preview.

Bug: 809738
Change-Id: I205c82a4dde46372b22dfa5e6d49b6c6022c0010
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508876
Reviewed-by: Alan Screen <awscreen@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822492}

chrome/services/printing/BUILD.gn

@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//printing/buildflags/buildflags.gni")
 import("//testing/test.gni")
 
 source_set("lib") {
@@ -16,13 +17,16 @@ source_set("lib") {
 
   deps = [
     "//components/crash/core/common:crash_key",
-    "//components/pwg_encoder",
     "//pdf",
     "//printing",
     "//printing/mojom",
     "//ui/gfx",
   ]
 
+  if (enable_print_preview) {
+    deps += [ "//components/pwg_encoder" ]
+  }
+
   public_deps = [
     "//base",
     "//chrome/services/printing/public/mojom",