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Server-side data can theoretically contain duplicate GUIDs, and the end
result today is a crash.

It is hard to deal with this case or even reason about what a reasonable
behavior is, so this patch instead detects and resolves the issue in a
preprocessing stage, such that offending GUIDs are ignored.

Doing so allows further refactorings to simplify the merge logic.

Bug: 978430
Change-Id: I4926bfe88aee2724633a447b0ed9ce086358b439
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1939974
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720217}