[Private Network Access] Disable preflights by default.

> Merging to 103 from ToT.

Anecdotal reports suggest that in some environments failed preflights
are leading to failed requests. This patch disables the feature while
we continue to investigate.

(cherry picked from commit dd306fb)

Bug: 1274149, 1329248
Change-Id: Id07fb24398627407ce98c4c6d424cf97242217bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3669044
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1007517}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3672354
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Auto-Submit: Mike West <mkwst@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/branch-heads/5060@{#293}
Cr-Branched-From: b83393d-refs/heads/main@{#1002911}

content/public/common/content_features.cc

@@ -696,7 +696,7 @@ const base::Feature kPrivateNetworkAccessRespectPreflightResults = {
 // Enables sending CORS preflight requests ahead of private network requests.
 // See: https://wicg.github.io/private-network-access/#cors-preflight
 const base::Feature kPrivateNetworkAccessSendPreflights = {
-    "PrivateNetworkAccessSendPreflights", base::FEATURE_ENABLED_BY_DEFAULT};
+    "PrivateNetworkAccessSendPreflights", base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Enable the ProactivelySwapBrowsingInstance experiment. A browsing instance
 // represents a set of frames that can script each other. Currently, Chrome does