Auto merge of #12685 - ConnorGBrewster:history_length, r=asajeffrey

Implement history.length

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- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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tests/wpt/metadata/html/browsers/history/the-history-interface/001.html.ini

@@ -1,8 +1,5 @@
 [001.html]
   type: testharness
-  [history.length should update when loading pages in an iframe]
-    expected: FAIL
-
   [history.length should update when setting location.hash]
     expected: FAIL
 

tests/wpt/metadata/html/browsers/history/the-history-interface/002.html.ini

@@ -1,8 +1,5 @@
 [002.html]
   type: testharness
-  [history.length should update when loading pages in an iframe]
-    expected: FAIL
-
   [history.length should update when setting location.hash]
     expected: FAIL
 

tests/wpt/metadata/html/browsers/history/the-location-interface/location_assign_about_blank.html.ini

@@ -0,0 +1,5 @@
+[location_assign_about_blank.html]
+  type: testharness
+  [location.assign with initial about:blank browsing context]
+    expected: FAIL
+

tests/wpt/metadata/html/dom/interfaces.html.ini

@@ -5598,9 +5598,6 @@
   [BarProp interface: attribute visible]
     expected: FAIL
 
-  [History interface: attribute length]
-    expected: FAIL
-
   [History interface: attribute state]
     expected: FAIL
 
@@ -5610,9 +5607,6 @@
   [History interface: operation replaceState(any,DOMString,DOMString)]
     expected: FAIL
 
-  [History interface: window.history must inherit property "length" with the proper type (0)]
-    expected: FAIL
-
   [History interface: window.history must inherit property "state" with the proper type (1)]
     expected: FAIL