Support clang-cl build on Windows.

third_party/third_party.gyp

Index: third_party/third_party.gyp
diff --git a/third_party/third_party.gyp b/third_party/third_party.gyp
index d58c9a71d015693e41e46bcc756c8d5135a88dc0..758d23ef0d0c7717a6f239b5f8952adfed972849 100644
--- a/third_party/third_party.gyp
+++ b/third_party/third_party.gyp
@@ -239,6 +239,12 @@
           ],
         }],
       ],
+      'variables': {
+        'clang_warning_flags': [
+          # Avoid warning for undefined behaviour.
+          '-Wno-shift-negative-value',
+        ],
+      }
     },
     {
       'target_name': 'fx_libopenjpeg',
@@ -322,7 +328,12 @@
           ],
         }],
       ],
-
+      'variables': {
+        'clang_warning_flags': [
+          # Avoid warning for undefined behaviour. https://crbug.com/507712
+          '-Wno-shift-negative-value',
+        ]
+      },
     },
     {
       'target_name': 'pdfium_base',