Make webkit_gpu build on Win64

webkit/gpu/webkit_gpu.gypi

Index: webkit/gpu/webkit_gpu.gypi
===================================================================
--- webkit/gpu/webkit_gpu.gypi	(revision 178856)
+++ webkit/gpu/webkit_gpu.gypi	(working copy)
@@ -51,6 +51,8 @@
       'defines': [
         'WEBKIT_GPU_IMPLEMENTATION',
       ],
+      # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
+      'msvs_disabled_warnings': [ 4267, ],
     },
   ],
 }