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This fixes the destructor to not (incorrectly) assert that it can be
called on any thread. The class comment correctly documents that this
class has to be destroyed on the file task runner all other methods
get invoked on as well.

Change-Id: I281d9886e8076abc0f77e86a91ad7b79fb140e71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346931
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796612}