PrecacheFetcher::CancelPrecaching should run in the main UI thread

components/precache/core/precache_fetcher.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/precache/core/precache_fetcher.h"
 
 #include <algorithm>
 #include <limits>
 #include <string>
 #include <utility>
@@ skipping 341 matching lines @@
     if (resource.has_url())
       resource_urls_to_fetch_.push_back(GURL(resource.url()));
   }
   unfinished_work_ = std::move(unfinished_work);
 }
 
 PrecacheFetcher::~PrecacheFetcher() {
 }
 
 std::unique_ptr<PrecacheUnfinishedWork> PrecacheFetcher::CancelPrecaching() {
+  // This could get called multiple times, and it should be handled gracefully.
+  if (!unfinished_work_)
+    return nullptr;
+
   unfinished_work_->clear_manifest();
   unfinished_work_->clear_resource();
   for (const auto& manifest : manifest_urls_to_fetch_)
     unfinished_work_->add_manifest()->set_url(manifest.spec());
   for (const auto& resource : resource_urls_to_fetch_)
     unfinished_work_->add_resource()->set_url(resource.spec());
   for (const auto& it : pool_.elements()) {
     const Fetcher* fetcher = it.first;
     if (fetcher->is_resource_request())
       unfinished_work_->add_resource()->set_url(fetcher->url().spec());
@@ skipping 195 matching lines @@
 
 void PrecacheFetcher::UpdateStats(int64_t response_bytes,
                                   int64_t network_response_bytes) {
   unfinished_work_->set_total_bytes(
       unfinished_work_->total_bytes() + response_bytes);
   unfinished_work_->set_network_bytes(
       unfinished_work_->network_bytes() + network_response_bytes);
 }
 
 }  // namespace precache