Add standard "Must be after no_chromium_code" comment in GN files.

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.
 
 import("//testing/test.gni")
 import("pdfium.gni")
 
 config("pdfium_config") {
   cflags = []
   include_dirs = [ "third_party/freetype/include" ]
@@ skipping 536 matching lines @@
     "core/src/fxge/ge/fx_ge_path.cpp",
     "core/src/fxge/ge/fx_ge_ps.cpp",
     "core/src/fxge/ge/fx_ge_text.cpp",
     "core/src/fxge/ge/text_int.h",
   ]
 
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     ":pdfium_config",
     "//build/config/compiler:no_chromium_code",
-    # As of this writing, this needs to be last to suppress a -Wswitch warning.
-    # https://crbug.com/547906
+    # Must be after no_chromium_code for warning flags to be ordered correctly.
     ":fxge_warnings",
   ]
 
   if (pdf_use_skia) {
     sources = [
       "core/src/fxge/skia/fx_skia_blitter_new.cpp",
       "core/src/fxge/skia/fx_skia_device.cpp",
     ]
     include_dirs = [
       "//third_party/skia/include/config",
@@ skipping 243 matching lines @@
       "//v8",
       "//v8/include",
     ]
   }
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     ":pdfium_config",
     "//build/config/compiler:no_chromium_code",
   ]
 }