Apply a patch to prevent an integer overflow in expat.

See https://www.mozilla.org/en-US/security/advisories/mfsa2015-54/ for Mozilla's advisory. Patch taken from https://hg.mozilla.org/releases/mozilla-esr31/rev/2f3e78643f5c

BUG=492052

Review URL: https://codereview.chromium.org/1151263010

Cr-Commit-Position: refs/heads/master@{#332964}