Enable RemovePurposeHeaderForPrefetch and Remove Purpose: prefetch header from tests.

Now that prefetches and prerenders are utilizing the Sec-Purpose header
for prefetches and prerenders, we will move to remove the legacy
Purpose: prefetch header that is still currently passed. This will be
behind a feature flag/ kill switch to prevent compat issues.

This will be scoped to speculation rules prefetch, speculation rules
prerender, <link rel=prefetch>, and Chromium's non-standard <link
rel=prerender>.

chromestatus: https://chromestatus.com/feature/5088012836536320
blink-dev:
https://groups.google.com/a/chromium.org/g/blink-dev/c/iBAJV-lgfLI

Bug: 420724819
Change-Id: I81dafe67a3d9eca72f7d6b0a75e66e921b5a0443
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6680595
Reviewed-by: Robert Flack <[email protected]>
Commit-Queue: Steven Wei <[email protected]>
Reviewed-by: Hiroki Nakagawa <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1498327}

Author: srwei

speculation-rules/prerender/resources/exec.py

@@ -2,8 +2,8 @@
 import os
 
 def main(request, response):
-    purpose = request.headers.get(b"purpose")
-    if (purpose == b'prefetch' and b"code" in request.GET):
+    sec_purpose = request.headers.get(b"sec-purpose")
+    if (sec_purpose == b'prefetch;prerender' and b"code" in request.GET):
         code = int(request.GET.first(b"code"))
     else:
         code = 200