Disable size_t to int conversion data loss warnings in media win64 builds

BUG=172415,166496
TEST=local build of media target on win64 doesn't emit compiler warning 4267, with libvpx DEPS rolled privately per https://codereview.chromium.org/12087013/
R=scherkus@chromium.org


Review URL: https://chromiumcodereview.appspot.com/12094047

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@179612 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed