Add chrome crash service to GN build on Windows.

components/crash/content/tools/BUILD.gn

 # 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.
 
-if (is_win) {
-  source_set("crash_service") {
-    sources = [
-      "crash_service.cc",
-      "crash_service.h",
-    ]
+assert(is_win)
 
-    deps = [
-      "//base",
-      "//breakpad:breakpad_handler",
-      "//breakpad:breakpad_sender",
-    ]
-  }
+source_set("crash_service") {
+  sources = [
+    "crash_service.cc",
+    "crash_service.h",
+  ]
+
+  deps = [
+    "//base",
+    "//breakpad:breakpad_handler",
+    "//breakpad:breakpad_sender",
+  ]
 }