Enable SignedExchangeSubresourcePrefetch at tip of tree

Intent to ship:
https://groups.google.com/a/chromium.org/g/blink-dev/c/tybtqf-tdB4/m/PSNj5vSEAwAJ

Bug: 935267
Change-Id: Ie690e7e814c5157050af520a5f35b492a3d80b4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415709Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808225}

content/public/common/content_features.cc

@@ -620,7 +620,7 @@ const base::Feature kSignedExchangeReportingForDistributors{
 // Subresource prefetching+loading via Signed HTTP Exchange
 // https://www.chromestatus.com/features/5126805474246656
 const base::Feature kSignedExchangeSubresourcePrefetch{
-    "SignedExchangeSubresourcePrefetch", base::FEATURE_DISABLED_BY_DEFAULT};
+    "SignedExchangeSubresourcePrefetch", base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Origin-Signed HTTP Exchanges (for WebPackage Loading)
 // https://www.chromestatus.com/features/5745285984681984