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The third_party/expat directory has had a checked in copy of bits of
libexpat. This has make updating quite difficult and blame more or less
impossible. Move to using expat from DEPS to make rolling easier and
more verifiable. This change should be build only and is not expected to
actually change the source compiled.

Bug: chromium:1004341
Change-Id: I7fd74ddb97e8f5302bd4dfe9b24a1bd20821cf55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1822703Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Ben Wagner <bungeman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700374}