Use chromium_code standard for PDFium GYP compilation

xfa.gyp

 {
   "variables": {
     "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,
     ],
-    'variables': {
-      'clang_warning_flags': [
-        # TODO(thestig): Fix all instances, remove this, pdfium:29
-        '-Wno-sign-compare',
-      ],
-      # Make sure Chromium's build/common.gypi doesn't re-add the flag on linux.
-      'cflags_cc!': [ '-Wsign-compare' ],
-    },
   },
   "targets":[
     {
       "target_name":"xfa",
       "type":"static_library",
       'include_dirs': [
         # This is implicit in GN.
         '<(DEPTH)',
         '.',
         'third_party/freetype/include',
@@ skipping 719 matching lines @@
           "sources": [],
         }],
         ["OS == 'mac'", {
           "configurations": {},
           "sources": [],
         }],
       ]
     }
   ]
 }