appcache: add 6 months to corrupted resource workaround date

Given the uncertainty around whether the probabilistic finch-controlled
feature can be rolled out to all users before the existing date, move
this date forward another six months.

This does overlap with the removal of AppCache and the end of the
AppCache origin trial (M90) and so hopefully between AppCache being
removed and the finch feature rollout, this is the last time this date
will need to be moved.

Bug: 582750
Change-Id: I8b21da18ba5a39632542aa8dad168ace15564afb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414988
Commit-Queue: enne <enne@chromium.org>
Auto-Submit: enne <enne@chromium.org>
Reviewed-by: Chase Phillips <cmp@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807651}

content/browser/appcache/appcache_update_job.cc

@@ -131,14 +131,20 @@ ResourceCheck CanUseExistingResource(
   // create a large number of expired cache entries, and the unnecessary
   // If-Modified-Since requests are causing noticeable levels of traffic.
   //
-  // The logic below is a workaround while a longer-term fix gets developed and
-  // deployed. We'll consider all cache entries with invalid times to have been
-  // created on Tue, Dec 31 2019.
+  // There is currently a Finch-controlled kAppCacheCorruptionRecoveryFeature
+  // that is turned on for some users.  Once this has been rolled out fully,
+  // then this workaround below can be removed.
+  //
+  // The logic below is a workaround to prevent refetching these corrupted
+  // cache entries while this kAppCacheCorruptionRecoveryFeature is rolled out
+  // to all users.  We'll consider all cache entries with invalid times to have
+  // been created on Tue, Jun 30 2020.  This date has been moved several times
+  // to prevent resources that haven't yet expired from being refetched all at
+  // once.
   //
-  // TODO(cmp): Add timeline info here.
   bool found_corruption = false;
   static constexpr base::Time::Exploded kInvalidTimePlaceholderExploded = {
-      2019, 12, 2, 31, 0, 0, 0, 0};
+      2020, 6, 2, 30, 0, 0, 0, 0};
   if (request_time.is_null()) {
     bool conversion_succeeded = base::Time::FromUTCExploded(
         kInvalidTimePlaceholderExploded, &request_time);