[whatwg/streams] A couple Bikeshed warnings (#929)

[From CI](https://travis-ci.org/whatwg/streams/builds/385245334):

```
\033[1;96mWARNING:\033[0m Unexported dfn that's not referenced locally - did you mean to export it?
<dfn data-dfn-for="underlying source" data-dfn-type="dfn" id="underlying-source-autoallocatechunksize" data-lt="autoAllocateChunkSize" data-noexport="by-default"><code>autoAllocateChunkSize</code><a class="self-link" href="#underlying-source-autoallocatechunksize"></a></dfn>
\033[1;96mWARNING:\033[0m Unexported dfn that's not referenced locally - did you mean to export it?
<dfn data-dfn-for="queuing strategy" data-dfn-type="dfn" id="queuing-strategy-highwatermark" data-lt="highWaterMark" data-noexport="by-default"><code>highWaterMark</code><a class="self-link" href="#queuing-strategy-highwatermark"></a></dfn>
\033[7;32m ✔ \033[0m Successfully generated, with warnings
```

Haven't looked at these much yet.

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