Disallow task observers on some BrowserThreads.

content/browser/browser_thread_impl.cc

Index: content/browser/browser_thread_impl.cc
diff --git a/content/browser/browser_thread_impl.cc b/content/browser/browser_thread_impl.cc
index 5657882dfa97ab16d0fc49f990194889abd97100..51b8c599417d987834530e8691b0c19c4ddc9e71 100644
--- a/content/browser/browser_thread_impl.cc
+++ b/content/browser/browser_thread_impl.cc
@@ -190,7 +190,9 @@ void BrowserThreadImpl::Init() {
       BrowserThread::CurrentlyOn(BrowserThread::FILE_USER_BLOCKING) ||
       BrowserThread::CurrentlyOn(BrowserThread::PROCESS_LAUNCHER) ||
       BrowserThread::CurrentlyOn(BrowserThread::CACHE)) {
-    base::MessageLoop::current()->DisallowNesting();
+    base::MessageLoop* message_loop = base::MessageLoop::current();
+    message_loop->DisallowNesting();
+    message_loop->DisallowTaskObservers();
   }
 
   using base::subtle::AtomicWord;