disable c4267 in libpng for building on win x64

R=jschuh@chromium.org
BUG=167187


Review URL: https://chromiumcodereview.appspot.com/11778076

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@175999 0039d316-1c4b-4281-b951-d872f2087c98

third_party/libpng/libpng.gyp

@@ -60,6 +60,8 @@
           'export_dependent_settings': [
             '../zlib/zlib.gyp:zlib',
           ],
+          # TODO(jschuh): http://crbug.com/167187
+          'msvs_disabled_warnings': [ 4267 ],
           'conditions': [
             ['OS!="win"', {'product_name': 'png'}],
             ['OS=="win"', {