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This patch replaces the "unknown" buffer map async status with
two more detailed ones (destroyed_before_callback and
ummapped_before_callback) to provide more information about
these abnormal buffer may async statuses.

BUG=dawn:533

Change-Id: I17480eb87b7af5ef131e37843ad37097ad4f5026
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2434110
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811640}