Support clang-cl build on Windows.

third_party/third_party.gyp

 # Copyright 2014 PDFium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'variables': {
     'pdf_enable_xfa%': 0,  # Set to 1 by standalone.gypi in standalone builds.
   },
   'target_defaults': {
     'defines': [
@@ skipping 221 matching lines @@
       'conditions': [
         ['os_posix==1', {
           'cflags': [
             '-Wno-main',
             '-Wno-missing-braces',
             '-Wno-shift-negative-value',
             '-Wno-unused',
           ],
         }],
       ],
+      'variables': {
+        'clang_warning_flags': [
+          '-Wno-shift-negative-value',

[Nico, 2016/03/09 02:30:19]

can you add a comment describing why this is needed?

[Oliver Chang, 2016/03/09 17:00:17]

Done.

+        ],
+      }
     },
     {
       'target_name': 'fx_libopenjpeg',
       'type': 'static_library',
       'sources': [
         'libopenjpeg20/bio.c',
         'libopenjpeg20/cio.c',
         'libopenjpeg20/dwt.c',
         'libopenjpeg20/event.c',
         'libopenjpeg20/function_list.c',
@@ skipping 63 matching lines @@
         'zlib_v128/zutil.c',
       ],
       'conditions': [
         ['os_posix==1', {
           'cflags': [
             # TODO(dsinclair): Remove if fixed upstream. https://crbug.com/507712
             '-Wno-shift-negative-value',
           ],
         }],
       ],
-
+      'variables': {
+        'clang_warning_flags': [
+          '-Wno-shift-negative-value',

[Nico, 2016/03/09 02:30:19]

likewise

[Oliver Chang, 2016/03/09 17:00:17]

Done.

+        ]
+      },
     },
     {
       'target_name': 'pdfium_base',
       'type': 'none',
       'sources': [
         'base/logging.h',
         'base/macros.h',
         'base/numerics/safe_conversions.h',
         'base/numerics/safe_conversions_impl.h',
         'base/numerics/safe_math.h',
@@ skipping 45 matching lines @@
             'libtiff/tif_version.c',
             'libtiff/tif_warning.c',
             'libtiff/tif_write.c',
             'libtiff/tif_zip.c',
           ],
         },
       ],
     }],
   ],
 }