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This CL add some CHECKs for debugging purpose to detect cases when
duplicate tombstones are tracked in the bookmarks tracker.

Bug: 963469
Change-Id: I2f0c23c5ece50264e9a7f07f2cf8abd956ff72d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1613182Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660053}