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This directory claims to be the sole, canonical source for this code,
so tracking changes in the README serves no real purpose.  Remove the
existing ones and add the directory to the PRESUBMIT exclusion list.

While there, this removes the long-gone third_party/WebKit directory
from the exclusion list and alphabetizes the list.

Bug: none
Change-Id: I9e2ab3145ea28c473a2e1c3424b4914ccc14ee30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2294017
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#787546}