Note that WritableStream with hwm 0 is bad

Add a comment to note that creating a WritableStream with a
high_water_mark of 0 will result in piping not working.

Change-Id: Iacf6769fa017b56f192789b02fb1ce13df4e4986
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1852347Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705113}

third_party/blink/renderer/core/streams/writable_stream.h

@@ -32,6 +32,10 @@ class CORE_EXPORT WritableStream : public ScriptWrappable {
                                 ScriptValue underlying_sink,
                                 ScriptValue strategy,
                                 ExceptionState&);
+
+  // Creates a WritableStream from C++. If |high_water_mark| is set to 0 then
+  // piping to this writable stream will not work, because there will always be
+  // backpressure. Usually 1 is the right choice.
   static WritableStream* CreateWithCountQueueingStrategy(
       ScriptState*,
       UnderlyingSinkBase*,