Fix two ContentHashFetcherTest.* tests' flakiness.

extensions/browser/content_hash_fetcher.cc

Index: extensions/browser/content_hash_fetcher.cc
diff --git a/extensions/browser/content_hash_fetcher.cc b/extensions/browser/content_hash_fetcher.cc
index b637ce903652ab28c274d43fc92696f60718b2d2..b7e0214d7af26c8322e1a56fe0f76d6958df303c 100644
--- a/extensions/browser/content_hash_fetcher.cc
+++ b/extensions/browser/content_hash_fetcher.cc
@@ -141,6 +141,7 @@ class ContentHashFetcherJob
   content::BrowserThread::ID creation_thread_;
 
   // Used for fetching content signatures.
+  // Created and destroyed on |creation_thread_|.
   std::unique_ptr<net::URLFetcher> url_fetcher_;
 
   // The key used to validate verified_contents.json.
@@ -218,6 +219,13 @@ bool ContentHashFetcherJob::IsCancelled() {
 }
 
 ContentHashFetcherJob::~ContentHashFetcherJob() {
+  // Destroy |url_fetcher_| on correct thread.
+  // It was possible for it to be deleted on blocking pool from
+  // MaybeCreateHashes(). See https://crbug.com/702300 for details.
+  if (url_fetcher_ && !content::BrowserThread::CurrentlyOn(creation_thread_)) {
+    content::BrowserThread::DeleteSoon(creation_thread_, FROM_HERE,
+                                       url_fetcher_.release());
+  }
 }
 
 bool ContentHashFetcherJob::LoadVerifiedContents(const base::FilePath& path) {
@@ -292,11 +300,14 @@ void ContentHashFetcherJob::OnURLFetchComplete(const net::URLFetcher* source) {
   VLOG(1) << "URLFetchComplete for " << extension_id_
           << " is_success:" << url_fetcher_->GetStatus().is_success() << " "
           << fetch_url_.possibly_invalid_spec();
+  // Delete |url_fetcher_| once we no longer need it.

[lazyboy, 2017/04/19 00:19:33]

As we discussed on vc, this is the deletion. However, I realized this isn't
enough, because ContentHashFetcherJob can be Cancel()ed from any thread.
Therefore I've also added
DeleteSoon to cover all the cases. This change isn't a requirement anymore, but
I thought
it would be good to do.

I've also thought of making ContentHashFetcherJob DeleteOnUI at first, but that
wouldn't
always work because we create ContentHashFetcherJob on dynamic creation_thread_,
which can or cannot be UI I think.

+  std::unique_ptr<net::URLFetcher> url_fetcher = std::move(url_fetcher_);
+
   if (IsCancelled())
     return;
   std::unique_ptr<std::string> response(new std::string);
-  if (!url_fetcher_->GetStatus().is_success() ||
-      !url_fetcher_->GetResponseAsString(response.get())) {
+  if (!url_fetcher->GetStatus().is_success() ||
+      !url_fetcher->GetResponseAsString(response.get())) {
     DoneFetchingVerifiedContents(false);
     return;
   }