Re-enable warning 4201 for xfa and other clean up

xfa.gyp

 {
   "variables": {
     "pdf_enable_v8%": 1,
   },
   "target_defaults": {
     "defines": [
       "PDF_ENABLE_XFA",
     ],
     'msvs_disabled_warnings': [
       4005, 4018, 4146, 4333, 4345, 4267,
-      # TODO(ochang): Investigate if this can be fixed properly.
-      4201,
       # 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' ],
@@ skipping 672 matching lines @@
           "sources": [],
         }],
         ["OS == 'mac'", {
           "configurations": {},
           "sources": [],
         }],
       ]
     }
   ]
 }