Enable chromium_code standard for XFA code

xfa.gyp

 {
   "variables": {
+    "chromium_code": 1,
     "pdf_enable_v8%": 1,
   },
   "target_defaults": {
     "defines": [
       "PDF_ENABLE_XFA",
     ],
     'msvs_disabled_warnings': [
-      4005, 4018, 4146, 4333, 4345, 4267,
-      # TODO(thestig): Fix all instances, remove this, pdfium:29
-      4245, 4310, 4389, 4701, 4702, 4706, 4800,
+      4267,
     ],
   },
   "targets":[
     {
       "target_name":"xfa",
       "type":"static_library",
       'include_dirs': [
         # This is implicit in GN.
         '<(DEPTH)',
         '.',
@@ skipping 669 matching lines @@
         "xfa/fxgraphics/cfx_pattern.cpp",
         "xfa/fxgraphics/cfx_pattern.h",
         "xfa/fxgraphics/cfx_shading.cpp",
         "xfa/fxgraphics/cfx_shading.h",
         "xfa/fxgraphics/include/cfx_graphics.h",
         "xfa/fxjse/include/fxjse.h",
       ],
       "conditions": [
         ["clang==1" , {
         }],
+        ["os_posix==1 and clang==0", { # When GCC
+          'cflags': [ '-Wno-error=strict-overflow' ],
+        }],
         ["pdf_enable_v8==1", {
           'dependencies': [
             '<(DEPTH)/v8/src/v8.gyp:v8',
           ],
           'export_dependent_settings': [
             '<(DEPTH)/v8/src/v8.gyp:v8',
           ],
           'include_dirs': [
             '<(DEPTH)/v8',
             '<(DEPTH)/v8/include',
@@ skipping 35 matching lines @@
           "sources": [],
         }],
         ["OS == 'mac'", {
           "configurations": {},
           "sources": [],
         }],
       ]
     }
   ]
 }