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Rather than depending on system provided rt.jar, check one into cipd and
use that one instead for hermetic builds. The one used is from
openjdk-8-jdk.

Add myself to OWNERS under //third_party/jdk and update README.chromium
with instructions for updating the extras subdirectory.

Bug: 693079
Change-Id: I65361bd11aa0edd60dceb102b45082848dac6244
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884367
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Auto-Submit: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710144}