Do not disable warnings-as-errors for pdfium.

It currently doesn't have any build warnings, and this way the
chromium build is guaranteed to stay warning-free after pdfium rolls.

BUG=none
R=jam@chromium.org

Review URL: https://codereview.chromium.org/373643002

pdfium.gyp

diff --git a/pdfium.gyp b/pdfium.gyp
index a78d104..466e9c3 100644
--- a/pdfium.gyp
+++ b/pdfium.gyp
@@ -1,6 +1,5 @@
 {
   'variables': {
-    'win_third_party_warn_as_error': 'false',
     'pdf_use_skia%': 0,
   },
   'target_defaults': {
@@ -37,14 +36,8 @@
       ['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