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The spec says that they have to, but we weren't checking it from what I
can see.

Bug: 827574
Change-Id: I96f1a7b5078cfc4c5e88647f47e3a60821901848
Reviewed-on: https://chromium-review.googlesource.com/982632
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548146}