Componentize spellcheck [1]: create component and move switches there.

components/spellcheck/common/BUILD.gn

Index: components/spellcheck/common/BUILD.gn
diff --git a/services/user/public/cpp/BUILD.gn b/components/spellcheck/common/BUILD.gn
similarity index 69%
copy from services/user/public/cpp/BUILD.gn
copy to components/spellcheck/common/BUILD.gn
index f341a18b19af81ef3555e0516a57d41519a3b99b..d9fa04b98b4fccf92b4b37bfbf6bc90c56c3dd02 100644
--- a/services/user/public/cpp/BUILD.gn
+++ b/components/spellcheck/common/BUILD.gn
@@ -2,9 +2,9 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-source_set("cpp") {
+source_set("common") {
   sources = [
-    "constants.cc",
-    "constants.h",
+    "spellcheck_switches.cc",
+    "spellcheck_switches.h",
   ]
 }