PrecacheFetcher::CancelPrecaching should run in the main UI thread

components/precache/core/precache_fetcher.cc

Index: components/precache/core/precache_fetcher.cc
diff --git a/components/precache/core/precache_fetcher.cc b/components/precache/core/precache_fetcher.cc
index dc0d3a272327a9ce1002ba33b377b2abb2f7e058..a5cf65086d2f665ced1263529f2e433788d25c9e 100644
--- a/components/precache/core/precache_fetcher.cc
+++ b/components/precache/core/precache_fetcher.cc
@@ -359,6 +359,10 @@ 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_)