[COOP] Access reporting. Fix the python server.

A python server is used has a communication channel in between documents
from different browsing context group and potentially cross-origin.

There are two methods:
- one for writing a message.
- one for reading the message.

I initially supposed we won't need to store more than one message in the
stash. It turns out there are tests doing two write in a row without any
read in the middle. It was throwing an exception when this
happened.

This patch makes the server to maintain a queue of messages instead. It
doesn't throw errors anymore.

Bug: chromium:1109194
Change-Id: I25f8dc2c2a032781a4527a0994e94e46f85e12c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317865Reviewed-by: Pâris Meuleman <pmeuleman@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791901}

third_party/blink/web_tests/external/wpt/html/cross-origin-opener-policy/reporting/resources/dispatcher.py

@@ -9,12 +9,17 @@ def main(request, response):
         return b''
 
     uuid = request.GET[b'uuid']
+    stashed = request.server.stash.take(uuid)
+    if stashed is None:
+        stashed = []
 
     if request.method == u'POST':
-        return request.server.stash.put(uuid, request.body)
+        stashed.append(request.body)
+        ret = b'done'
     else:
-        body = request.server.stash.take(uuid)
-        if body is None:
-            return b'not ready'
+        if len(stashed) == 0:
+            ret = b'not ready'
         else:
-            return body
+            ret = stashed.pop(0)
+    request.server.stash.put(uuid, stashed)
+    return ret;