Clarify creation of ReadableStream objects

Follow-ups:

* w3c/ServiceWorker#1338
* WICG/background-fetch#104

Author: jyasskin

Diff for: fetch.bs

@@ -1872,22 +1872,33 @@ we define common operations for {{ReadableStream}} objects. [[!STREAMS]]
 
 <p>To
 <dfn export for=ReadableStream id=concept-construct-readablestream>construct a <code>ReadableStream</code> object</dfn>
-with given <var>strategy</var>, <var>pull</var> action and <var>cancel</var> action, all of which
-are optional, run these steps:
+optionally with a <var>highWaterMark</var>, <var>sizeAlgorithm</var> algorithm, <var>pull</var>
+action, and <var>cancel</var> action, run these steps:
+
+<p class=note>This algorithm used to take a <var ignore>strategy</var> parameter, whose
+<code>highWaterMark</code> and <code>sizeAlgorithm</code> members were extracted to provide what are
+now separate parameters. If another specification still passes that <var ignore>strategy</var>
+parameter, please update it.
 
 <ol>
- <li><p>Let <var>init</var> be a new object.
+ <li><p>Let <var>startAlgorithm</var> be an algorithm that returns undefined.
+
+ <li><p>If <var>pull</var> is absent, then set it to an action that returns undefined.
+
+ <li><p>Let <var>pullAlgorithm</var> be an algorithm that returns the result of [=promise-calling=]
+ <var>pull</var>().
 
- <li><p>Set <var>init</var>["pull"] to a function that runs <var>pull</var> if <var>pull</var> is
- given.
+ <li><p>If <var>cancel</var> is absent, then set it to an action that returns undefined.
 
- <li><p>Set <var>init</var>["cancel"] to a function that runs <var>cancel</var> if
- <var>cancel</var> is given.
+ <li><p>Let <var>cancelAlgorithm</var> be an algorithm that takes a <var>reason</var> and returns
+ the result of [=promise-calling=] <var>cancel</var>(<var>reason</var>).
 
- <li><p>Let <var>stream</var> be the result of calling the initial value of {{ReadableStream}} as
- constructor with <var>init</var> and <var>strategy</var> if given.
+ <li><p>If <var>highWaterMark</var> is absent, then set it to 1.
 
- <li><p>Return <var>stream</var>.
+ <li><p>If <var>sizeAlgorithm</var> is absent, then set it to an algorithm that returns 1.
+
+ <li><p>Return [$CreateReadableStream$](<var>startAlgorithm</var>, <var>pullAlgorithm</var>,
+ <var>cancelAlgorithm</var>, <var>highWaterMark</var>, <var>sizeAlgorithm</var>).
 </ol>
 
 <p>To
@@ -4123,11 +4134,10 @@ Range Requests</cite>. [[HTTP-RANGE]] However, this is not widely supported by b
    <li><p>Return a <a>network error</a>.
   </ol>
 
- <li>
-  <p>Let <var>strategy</var> be an object. The user agent may choose any object.
+ <li><p>Let <var>highWaterMark</var> be a non-negative, non-NaN number, chosen by the user agent.
 
-  <p class="note no-backref"><var>strategy</var> is used to control the queuing strategy of
-  <var>stream</var> constructed below.
+ <li><p>Let <var>sizeAlgorithm</var> be an algorithm that accepts a <a>chunk</a> object and returns
+ a non-negative, non-NaN, non-infinite number, chosen by the user agent.
 
  <li><p>Let <var>pull</var> be an action that <a lt=resumed for=fetch>resumes</a> the ongoing fetch
  if it is <a lt=suspend for=fetch>suspended</a>.
@@ -4137,9 +4147,10 @@ Range Requests</cite>. [[HTTP-RANGE]] However, this is not widely supported by b
 
  <li>
   <p>Let <var>stream</var> be the result of
-  <a lt="construct a ReadableStream object" for=ReadableStream>constructing</a> a
-  {{ReadableStream}} object with <var>strategy</var>,
-  <var>pull</var> and <var>cancel</var>.
+  <a lt="construct a ReadableStream object" for=ReadableStream>constructing</a> a {{ReadableStream}}
+  object with <var>highWaterMark</var>, <var>sizeAlgorithm</var>, <var>pull</var>, and
+  <var>cancel</var>.
+
   <p class="note no-backref">This construction operation will not throw an exception.
 
  <li>