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This makes it much easier to roll expat. It automates calling rolldep,
updating the README.chromium, updating the expat_config.h
configuration header, and adds the OWNERS to the review.

Bug: chromium:1004341
Change-Id: I9e19e0016cedc12b1f2079cd54b95bd7e3d3f25a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829677Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Ben Wagner <bungeman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702869}