AppCache: Update mitigation date for corrupted caches

This CL moves the date assumed for corrupted caches from Tue Jan 29,
2019 to Sun Jul 6, 2019.

We expect this change to land in master and be backported to M78.
The CL currently has placeholder text where information about the
affected regions of response timestamp vs max-age/TTL will go in
a later change.

AppCache is currently deprecated with removal targeted for M82.  We
expect that either we'll deploy a more permanent solution by M82
or AppCache will be removed in that milestone.

Bug: 1014794
Change-Id: I98fb5042aea144048f82d686025ff7d84ee10ed5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1860143
Commit-Queue: Chase Phillips <cmp@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Auto-Submit: Chase Phillips <cmp@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706288}

content/browser/appcache/appcache_update_job.cc

@@ -116,11 +116,11 @@ bool CanUseExistingResource(const net::HttpResponseInfo* http_info) {
   //
   // 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, Jan 29 2019. This is the day when M72 was released to
-  // Chrome's Stable channel, and was chosen because M72 had the first large
-  // Network Service deployment.
+  // created on Sun, Jun 16 2019.
+  //
+  // TODO(cmp): Add timeline info here.
   static constexpr base::Time::Exploded kInvalidTimePlaceholderExploded = {
-      2019, 1, 2, 29, 0, 0, 0, 0};
+      2019, 7, 0, 7, 0, 0, 0, 0};
   constexpr base::Time default_initialized_time;
   if (request_time == default_initialized_time) {
     bool conversion_succeeded = base::Time::FromUTCExploded(