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Update gpu_testing_bot_details.md

- No longer sharing GCEs with Chromium in chromium.star
- tools/build repo restructure
- No more mb_config_buckets.pyl

Bug: 1123215
Change-Id: I3874ba6a63873dc560837c87372ee22a6a7a061a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2444440Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813284}