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Because of a finch trial of DefaultPassthroughCommandDecoder, a few bots
which were relying on the default to test the validating command decoder
ended up switching to passthrough. This CL reconfigures to be more
explicit

TBR=kbr@chromium.org

Bug: 1014368
Bug: 1014370
Change-Id: I5ca6c380755d529ce506e86ececbf070f472bd33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863152
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706087}