Add Request's isHistoryNavigation

And also a member on the request concept (history-navigation flag) to support this API. See w3c/ServiceWorker#1167 for the discussion that led to this.

Tests: web-platform-tests/wpt#10909.

Corresponding HTML change: whatwg/html#3674.

fetch.bs

@@ -1150,6 +1150,12 @@ Unless stated otherwise, it is unset.
 
 <p class="note no-backref">This flag is for exclusive use by HTML's navigate algorithm. [[!HTML]]
 
+<p>A <a for=/>request</a> has an associated
+<dfn export for=request id=concept-request-history-navigation-flag>history-navigation flag</dfn>.
+Unless stated otherwise, it is unset.
+
+<p class="note no-backref">This flag is for exclusive use by HTML's navigate algorithm. [[!HTML]]
+
 <hr>
 
 <p>A <a for=/>request</a> has an associated
@@ -4976,6 +4982,7 @@ interface Request {
   readonly attribute DOMString integrity;
   readonly attribute boolean keepalive;
   readonly attribute boolean isReloadNavigation;
+  readonly attribute boolean isHistoryNavigation;
   readonly attribute AbortSignal signal;
 
   [NewObject] Request clone();
@@ -5136,6 +5143,10 @@ initially a new {{AbortSignal}} object.
  <dt><code><var>request</var> . <a attribute for=Request>isReloadNavigation</a></code>
  <dd>Returns a boolean indicating whether or not <var>request</var> is for a reload navigation.
 
+ <dt><code><var>request</var> . <a attribute for=Request>isHistoryNavigation</a></code>
+ <dd>Returns a boolean indicating whether or not <var>request</var> is for a history
+ navigation (a.k.a. back-foward navigation).
+
  <dt><code><var>request</var> . <a attribute for=Request>signal</a></code>
  <dd>Returns the signal associated with <var>request</var>, which is an
  {{AbortSignal}} object indicating whether or not <var>request</var> has been aborted, and its abort
@@ -5239,10 +5250,11 @@ constructor must run these steps:
  <a for=request>integrity metadata</a> is
  <var>request</var>'s
  <a for=request>integrity metadata</a>,
- <a>keepalive flag</a> is <var>request</var>'s <a>keepalive flag</a>, and
+ <a>keepalive flag</a> is <var>request</var>'s <a>keepalive flag</a>,
  <a for=request>reload-navigation flag</a> is <var>request</var>'s
- <a for=request>reload-navigation flag</a>.
-
+ <a for=request>reload-navigation flag</a>, and
+ <a for=request>history-navigation flag</a> is <var>request</var>'s
+ <a for=request>history-navigation flag</a>.
 
  <li>
   <p>If any of <var>init</var>'s members are present, then:
@@ -5254,6 +5266,8 @@ constructor must run these steps:
 
    <li><p>Unset <var>request</var>'s <a for=request>reload-navigation flag</a>.
 
+   <li><p>Unset <var>request</var>'s <a for=request>history-navigation flag</a>.
+
    <li><p>Set <var>request</var>'s <a for=request>referrer</a> to
    "<code>client</code>"
 
@@ -5534,6 +5548,10 @@ is set, and false otherwise.
 invoked, must return true if the <a>context object</a>'s <a for=Request>request</a>'s
 <a for=request>reload-navigation flag</a> is set, and false otherwise.
 
+<p>The <dfn attribute for=Request><code>isHistoryNavigation</code></dfn> attribute's getter, when
+invoked, must return true if the <a>context object</a>'s <a for=Request>request</a>'s
+<a for=request>history-navigation flag</a> is set, and false otherwise.
+
 <p>The <dfn attribute for=Request><code>signal</code></dfn> attribute's getter must return the
 associated <a for="Request">signal</a>.