Fix a null AppCacheBackendImpl ptr crash in CompleteCrossSiteTransfer and re…

content/browser/appcache/appcache_request_handler.cc

Index: content/browser/appcache/appcache_request_handler.cc
diff --git a/content/browser/appcache/appcache_request_handler.cc b/content/browser/appcache/appcache_request_handler.cc
index c02223752afa7f823b2032d687eebc0bddf8b8ad..0a00878df3ccc412c80d9477e16b299faeb17646 100644
--- a/content/browser/appcache/appcache_request_handler.cc
+++ b/content/browser/appcache/appcache_request_handler.cc
@@ -187,6 +187,7 @@ void AppCacheRequestHandler::PrepareForCrossSiteTransfer(int old_process_id) {
   if (!host_)
     return;
   AppCacheBackendImpl* backend = host_->service()->GetBackend(old_process_id);
+  DCHECK(backend) << "appcache detected likely storage partition mismatch";
   old_process_id_ = old_process_id;
   old_host_id_ = host_->host_id();
   host_for_cross_site_transfer_ = backend->TransferHostOut(host_->host_id());
@@ -199,6 +200,7 @@ void AppCacheRequestHandler::CompleteCrossSiteTransfer(
     return;
   DCHECK_EQ(host_, host_for_cross_site_transfer_.get());
   AppCacheBackendImpl* backend = host_->service()->GetBackend(new_process_id);
+  DCHECK(backend) << "appcache detected likely storage partition mismatch";
   backend->TransferHostIn(new_host_id,
                           std::move(host_for_cross_site_transfer_));
 }