Remove stashed request headers for speculative parser tests

The old code would append all of the request headers for the request
to the stashed value. That causes the failure string for all dependent
tests to change when, e.g. the User-Agent string changes due to the
test being run on a different bot.

This CL removes the headers, as those seem only useful for debugging,
and are causing lots of flaky behavior on at least Chromium bots.

Prior failure message:
 FAIL Speculative parsing, document.write(): link-rel-stylesheet-disabled Unhandled rejection: assert_equals: speculative case incorrectly fetched expected "" but got "param-encodingcheck: %C4%9E\r\nHost: web-platform.test:8001\nConnection: keep-alive\nUser-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/999.77.34.5 Safari/537.36\nAccept: text/css,*/*;q=0.1\nReferer: http://web-platform.test:8001/html/syntax/speculative-parsing/generated/document-write/link-rel-stylesheet-disabled.tentative.sub.html\nAccept-Encoding: gzip, deflate\nAccept-Language: en-us,en\n\n"

New failure message:
 FAIL Speculative parsing, document.write(): link-rel-stylesheet-disabled Unhandled rejection: assert_equals: speculative case incorrectly fetched expected "" but got "param-encodingcheck: %C4%9E\r\n"

Fixed: 1250457,1249920,1249921,1249954,1250003,1250004
Bug: 1144176
Change-Id: I45dbbb8794c6bf2d5e962cecf531e06a7af09160
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3170927
Commit-Queue: Mason Freed <[email protected]>
Auto-Submit: Mason Freed <[email protected]>
Reviewed-by: Joey Arhar <[email protected]>
Cr-Commit-Position: refs/heads/main@{#923491}

Author: mfreed7

html/syntax/speculative-parsing/resources/stash.py

@@ -1,6 +1,6 @@
 def main(request, response):
     if request.GET[b"action"] == b"put":
         encodingcheck = u"param-encodingcheck: " + request.url_parts.query.split(u"&encodingcheck=")[1] + u"\r\n"
-        request.server.stash.put(request.GET[b"uuid"], encodingcheck + str(request.raw_headers))
+        request.server.stash.put(request.GET[b"uuid"], encodingcheck)
         return u''
     return request.server.stash.take(request.GET[b"uuid"])