Match GN and GYP compilation warning flags

For GN build:
Enable chromium_code compilation config for XFA code;
Remove sign-compare suppression;
Limit the strict-overflow warning suppression only to xfa target;

For GYP build:
Remove warning suppression 4800 from GYP non third-party code;

After this, all the warning flags and levels for GN and GYP
builds should match.

BUG=pdfium:29, pdfium:475

Review-Url: https://codereview.chromium.org/2011303003
diff --git a/BUILD.gn b/BUILD.gn
index 580bc57..63ac287 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -44,14 +44,12 @@
   if (is_win) {
     cflags += [ "/wd4267" ]
   }
+}
 
-  if (is_clang || is_posix) {
-    # TODO(thestig): Fix all instances, remove this, pdfium:29
-    cflags += [ "-Wno-sign-compare" ]
-  }
-
+config("xfa_warnings") {
+  visibility = [ ":*" ]
   if (is_posix && !is_clang) {  # When GCC.
-    cflags += [ "-Wno-strict-overflow" ]
+    cflags = [ "-Wno-strict-overflow" ]
   }
 }
 
@@ -888,11 +886,7 @@
     deps = [
       ":xfa",
     ]
-    configs -= [ "//build/config/compiler:chromium_code" ]
-    configs += [
-      ":pdfium_config",
-      "//build/config/compiler:no_chromium_code",
-    ]
+    configs += [ ":pdfium_config" ]
   }
 
   static_library("xfa") {
@@ -1592,10 +1586,9 @@
         "//v8:v8_libplatform",
       ]
     }
-    configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [
       ":pdfium_config",
-      "//build/config/compiler:no_chromium_code",
+      ":xfa_warnings",
     ]
   }
 }
diff --git a/build_gyp/standalone.gypi b/build_gyp/standalone.gypi
index f191c28..aa3feea 100644
--- a/build_gyp/standalone.gypi
+++ b/build_gyp/standalone.gypi
@@ -205,8 +205,7 @@
       # This section is PDFium specific.
       # ####
 
-      # C4800: forcing value to bool 'true' or 'false' (performance warning)
-      4800,
+      # Now there is no PDFium specific flag. Add here if needed.
 
       # ####
       # This section should match Chromium's build/common.gypi.
@@ -479,6 +478,10 @@
             '-Wno-unused-variable',
           ],
         },
+        'msvs_disabled_warnings': [
+          # forcing value to bool 'true' or 'false' (performance warning)
+          4800,
+        ],
         'msvs_settings': {
           'VCCLCompilerTool': {
             'WarningLevel': '3',