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This allows fuzzing to deserialize arbitrary messages, while still
checking programming errors.

Bug: 1046895
Change-Id: Iafc5418ddbcbf8197ce3991d08c4f8d7bf374234
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031165
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#737091}