Add a verification to check that HashTable is not modifiled while deleting buckets
Add a verification to check that HashTable is not modifiled while deleting buckets

If HashTable is modified or looked up while calling some destructor in deleteBucket(),
it will break the HashTable. This CL adds a verification about it.
I suspect that this is the cause of bug 531291.

BUG=531291

Review URL: https://codereview.chromium.org/1416633007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#355521}

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