[Merge M68]Check if weak_system_store object is valid before using it.

This may have caused the crash in crbug/847026

Bug: 847026
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ia3b4a4a756aec2898f8933173a61bdb097f935b5
Reviewed-on: https://chromium-review.googlesource.com/1076672
Reviewed-by: Eugene But <[email protected]>
Commit-Queue: Mohammad Refaat <[email protected]>
Cr-Original-Commit-Position: refs/heads/master@{#562573}(cherry picked from commit 2e6de02)
Reviewed-on: https://chromium-review.googlesource.com/1081068
Cr-Commit-Position: refs/branch-heads/3440@{#69}
Cr-Branched-From: 010ddcf-refs/heads/master@{#561733}

Author: mrefaat88

ios/net/cookies/cookie_store_ios.mm

@@ -556,13 +556,18 @@ bool HasExplicitDomain(const std::string& cookie_line) {
       system_store_->GetWeakPtr();
   system_store_->GetAllCookiesAsync(
       base::BindBlockArc(^(NSArray<NSHTTPCookie*>* cookies) {
+        if (!weak_system_store) {
+          if (!shared_callback.is_null())
+            std::move(shared_callback).Run(0);
+          return;
+        }
         int to_delete_count = 0;
         for (NSHTTPCookie* cookie in cookies) {
           base::Time creation_time =
               weak_system_store->GetCookieCreationTime(cookie);
           CanonicalCookie cc =
               CanonicalCookieFromSystemCookie(cookie, creation_time);
-          if (weak_system_store && shared_delete_info.Matches(cc)) {
+          if (shared_delete_info.Matches(cc)) {
             weak_system_store->DeleteCookieAsync(
                 cookie, SystemCookieStore::SystemCookieCallback());
             to_delete_count++;