Auto merge of #22514 - jdm:source, r=Manishearth

Implement source for postMessage events

Also make similar-origin iframes access their newly loaded document as soon as the new document is created. This should allow tests that check the event.source property to run correctly.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #22499 and fix #12715 and fix #22514.
- [x] There are tests for these changes

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tests/wpt/metadata/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/bailout-exception-vs-return-origin.sub.window.js.ini

@@ -12,6 +12,3 @@
   [document.open should throw a SecurityError with cross-origin document even when the ignore-opens-during-unload counter is greater than 0 (during pagehide event)]
     expected: FAIL
 
-  [document.open should throw a SecurityError with cross-origin document even when there is an active parser executing script]
-    expected: FAIL
-

tests/wpt/metadata/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/bailout-exception-vs-return-xml.window.js.ini

@@ -9,6 +9,3 @@
   [document.open should throw an InvalidStateError with XML document even when the ignore-opens-during-unload counter is greater than 0 (during pagehide event)]
     expected: FAIL
 
-  [document.open should throw an InvalidStateError with XML document even when there is an active parser executing script]
-    expected: FAIL
-

tests/wpt/metadata/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/bailout-side-effects-synchronous-script.window.js.ini

@@ -1,4 +0,0 @@
-[bailout-side-effects-synchronous-script.window.html]
-  [document.open bailout should not have any side effects (active parser whose script nesting level is greater than 0)]
-    expected: FAIL
-