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

third_party/BUILD.gn

 # Copyright 2014 PDFium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 group("third_party") {
   deps = [
     ":bigint",
     ":fx_freetype",
     ":pdfium_base",
   ]
@@ skipping 28 matching lines @@
   }
 }
 
 # Tests may link against this even if the production library doesn't,
 # so it needs to be separate from it.
 static_library("fx_freetype") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
     "//third_party/pdfium:pdfium_config",
+    # Must be after no_chromium_code for warning flags to be ordered correctly.
     ":fx_freetype_warnings",
   ]
   defines = [ "FT2_BUILD_LIBRARY" ]
   sources = [
     "freetype/include/freetype.h",
     "freetype/include/ft2build.h",
     "freetype/include/ftmm.h",
     "freetype/include/ftotval.h",
     "freetype/include/ftoutln.h",
     "freetype/include/internal/ftobjs.h",
@@ skipping 29 matching lines @@
     # calc_butt_cap() in agg_vcgen_stroke.cpp is unused.
     cflags = [ "-Wno-unused-function" ]
   }
 }
 
 source_set("fx_agg") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
     "//third_party/pdfium:pdfium_config",
+    # Must be after no_chromium_code for warning flags to be ordered correctly.
     ":fx_agg_warnings",
   ]
   sources = [
     "agg23/agg_basics.h",
     "agg23/agg_clip_liang_barsky.h",
     "agg23/agg_conv_dash.h",
     "agg23/agg_conv_stroke.h",
     "agg23/agg_curves.cpp",
     "agg23/agg_curves.h",
     "agg23/agg_path_storage.cpp",
@@ skipping 19 matching lines @@
       "-Wno-unused-function",
     ]
   }
 }
 
 source_set("fx_lcms2") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
     "//third_party/pdfium:pdfium_config",
+    # Must be after no_chromium_code for warning flags to be ordered correctly.
     ":fx_lcms2_warnings",
   ]
   sources = [
     "lcms2-2.6/include/lcms2.h",
     "lcms2-2.6/include/lcms2_plugin.h",
     "lcms2-2.6/src/cmscam02.c",
     "lcms2-2.6/src/cmscgats.c",
     "lcms2-2.6/src/cmscnvrt.c",
     "lcms2-2.6/src/cmserr.c",
     "lcms2-2.6/src/cmsgamma.c",
@@ skipping 150 matching lines @@
     "base/macros.h",
     "base/nonstd_unique_ptr.h",
     "base/numerics/safe_conversions.h",
     "base/numerics/safe_conversions_impl.h",
     "base/numerics/safe_math.h",
     "base/numerics/safe_math_impl.h",
     "base/stl_util.h",
     "base/template_util.h",
   ]
 }