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

R=thakis@chromium.org

Review URL: https://codereview.chromium.org/1411563004 .
diff --git a/BUILD.gn b/BUILD.gn
index abd189b..6accde5 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -554,8 +554,7 @@
   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",
   ]
 
diff --git a/third_party/BUILD.gn b/third_party/BUILD.gn
index f635814..e9e50cd 100644
--- a/third_party/BUILD.gn
+++ b/third_party/BUILD.gn
@@ -46,6 +46,7 @@
   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" ]
@@ -95,6 +96,7 @@
   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 = [
@@ -134,6 +136,7 @@
   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 = [