Revert the changes to pdfium.gyp in 4db62f5. To completely fix all the errors on all platforms we'd have to make changes to third party libraries. Some of those libraries are already shipped in chrome, and if we are going to want to share them we don't want them to diverge.
diff --git a/pdfium.gyp b/pdfium.gyp
index 732c2ab..8cf3950 100644
--- a/pdfium.gyp
+++ b/pdfium.gyp
@@ -1,5 +1,6 @@
 {
   'variables': {
+    'win_third_party_warn_as_error': 'false',
     'pdf_use_skia%': 0,
   },
   'target_defaults': {
@@ -36,8 +37,14 @@
       ['OS=="mac"', {
         'xcode_settings': {
           'GCC_ENABLE_CPP_EXCEPTIONS': 'YES',
+          'GCC_TREAT_WARNINGS_AS_ERRORS': 'NO',    # -Werror
         },
       }],
+      ['clang==1', {

+         'cflags!': [
+           '-Werror',
+         ],
+      }],
     ],
     'msvs_disabled_warnings': [
       4005, 4018, 4146, 4333, 4345