Turn on chromium_code (except -Wsign-compare, for now) for pdfium

pdfium.gyp

 {
   'variables': {
+    'chromium_code': 1,
     'pdf_use_skia%': 0,
     'pdf_enable_v8%': 1,
     'conditions': [
       ['OS=="linux"', {
         'bundle_freetype%': 0,
       }, {  # On Android there's no system FreeType. On Windows and Mac, only a
             # few methods are used from it.
         'bundle_freetype%': 1,
       }],
     ],
@@ skipping 22 matching lines @@
           }],
           ['target_arch=="ia32"', {
             'defines' : [ '_FX_CPU_=_FX_X86_', ],
           }],
         ],
       }],
     ],
     'msvs_disabled_warnings': [
       4005, 4018, 4146, 4333, 4345, 4267
     ],
+    'variables': {
+      'clang_warning_flags': [
+        # TODO(thestig): Fix all instances, remove this, pdfium:29
+        '-Wno-sign-compare',
+      ],
+    },
   },
   'targets': [
     {
       'target_name': 'pdfium',
       'type': 'static_library',
       'dependencies': [
         'third_party/third_party.gyp:bigint',
         'third_party/third_party.gyp:pdfium_base',
         'fdrm',
         'fpdfdoc',
@@ skipping 726 matching lines @@
           ],
           'dependencies': [
             '<(DEPTH)/v8/tools/gyp/v8.gyp:v8',
             '<(DEPTH)/v8/tools/gyp/v8.gyp:v8_libplatform',
           ],
         }],
       ],
     },
   ],
 }