Make SpellCheckPanel compiled and used only on Mac

components/spellcheck/renderer/BUILD.gn

Index: components/spellcheck/renderer/BUILD.gn
diff --git a/components/spellcheck/renderer/BUILD.gn b/components/spellcheck/renderer/BUILD.gn
index 1c3e482f24d6aebd5812c59a954c67be0a608e00..60f66ab86465d39c74d2874ae4757b21e3da5092 100644
--- a/components/spellcheck/renderer/BUILD.gn
+++ b/components/spellcheck/renderer/BUILD.gn
@@ -25,6 +25,7 @@ source_set("renderer") {
     "spelling_engine.h",
   ]
 
+  # TODO(xiaochengh): Negate these checks so that each file is listed only once.
   if (!use_browser_spellchecker) {
     sources -= [
       "platform_spelling_engine.cc",
@@ -32,6 +33,13 @@ source_set("renderer") {
     ]
   }
 
+  if (!has_spellcheck_panel) {
+    sources -= [
+      "spellcheck_panel.cc",
+      "spellcheck_panel.h",
+    ]
+  }
+
   if (is_android) {
     sources -= [
       "hunspell_engine.cc",