AppCache: Enable correct 304 handling by default.

We'll monitor the fix as it goes through the Chrome release waterfall,
instead of doing a Finch-based trial. This frees us from having to
reason about enterprises that may disable Finch.

Bug: 989611
Change-Id: I50389c12d31f63fa6257d879f3d840ffde2cb27e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2052245
Commit-Queue: Victor Costan <pwnall@chromium.org>
Commit-Queue: Chase Phillips <cmp@chromium.org>
Auto-Submit: Victor Costan <pwnall@chromium.org>
Reviewed-by: Chase Phillips <cmp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740894}

content/browser/appcache/appcache_update_job.cc

@@ -211,7 +211,7 @@ const base::Feature kAppCacheManifestScopeChecksFeature{
     "AppCacheManifestScopeChecks", base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kAppCacheUpdateResourceOn304Feature{
-    "AppCacheUpdateResourceOn304", base::FEATURE_DISABLED_BY_DEFAULT};
+    "AppCacheUpdateResourceOn304", base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Helper class for collecting hosts per frontend when sending notifications
 // so that only one notification is sent for all hosts using the same frontend.