Migrate ContentHashFetcher off of BlockingPool

R=rdevlin.cronin@chromium.org

Bug: 667892
Change-Id: I21df3c5a2eac7d6a5d2b1a3bc0322d752baca286
Reviewed-on: https://chromium-review.googlesource.com/663767
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501607}

extensions/browser/content_hash_fetcher.cc

@@ -26,6 +26,7 @@
 #include "extensions/browser/computed_hashes.h"
 #include "extensions/browser/content_hash_tree.h"
 #include "extensions/browser/content_verifier_delegate.h"
+#include "extensions/browser/extension_file_task_runner.h"
 #include "extensions/browser/verified_contents.h"
 #include "extensions/common/constants.h"
 #include "extensions/common/extension.h"
@@ -350,10 +351,8 @@ void ContentHashFetcherJob::DoneFetchingVerifiedContents(bool success) {
     return;
   }
 
-  content::BrowserThread::PostBlockingPoolSequencedTask(
-      "ContentHashFetcher",
-      FROM_HERE,
-      base::Bind(&ContentHashFetcherJob::MaybeCreateHashes, this));
+  GetExtensionFileTaskRunner()->PostTask(
+      FROM_HERE, base::Bind(&ContentHashFetcherJob::MaybeCreateHashes, this));
 }
 
 void ContentHashFetcherJob::MaybeCreateHashes() {