Downloads: replace BrowserThread::FILE with task scheduler.

content/browser/download/mhtml_generation_browsertest.cc

Index: content/browser/download/mhtml_generation_browsertest.cc
diff --git a/content/browser/download/mhtml_generation_browsertest.cc b/content/browser/download/mhtml_generation_browsertest.cc
index e13a93d2d6e825953dea97180d58fbd63c4b6962..b1817eb2bf774294443e8764d9f444d58b519ff8 100644
--- a/content/browser/download/mhtml_generation_browsertest.cc
+++ b/content/browser/download/mhtml_generation_browsertest.cc
@@ -15,6 +15,7 @@
 #include "base/strings/utf_string_conversions.h"
 #include "base/test/histogram_tester.h"
 #include "base/threading/thread_restrictions.h"
+#include "content/browser/download/download_task_runner.h"
 #include "content/browser/renderer_host/render_process_host_impl.h"
 #include "content/common/frame_messages.h"
 #include "content/public/browser/mhtml_extra_parts.h"
@@ -336,10 +337,9 @@ class GenerateMHTMLAndExitRendererMessageFilter : public BrowserMessageFilter {
   };
 
   void TaskX() {
-    BrowserThread::PostTask(
-        BrowserThread::FILE, FROM_HERE, base::Bind(
-            &GenerateMHTMLAndExitRendererMessageFilter::TaskY,
-            base::Unretained(this)));
+    GetDownloadTaskRunner()->PostTask(
+        FROM_HERE, base::Bind(&GenerateMHTMLAndExitRendererMessageFilter::TaskY,
+                              base::Unretained(this)));
   }
 
   void TaskY() {