Add TestBrowserThreadBundle into RenderViewHostTestHarness. Kill some unnecessary real threads.

webkit/browser/appcache/appcache_storage_impl.cc

Index: webkit/browser/appcache/appcache_storage_impl.cc
diff --git a/webkit/browser/appcache/appcache_storage_impl.cc b/webkit/browser/appcache/appcache_storage_impl.cc
index 23ee4bc448e8401d13d9289ae31d0db781251bae..259d3109161ed3eca5d7992554dc54583dec65fc 100644
--- a/webkit/browser/appcache/appcache_storage_impl.cc
+++ b/webkit/browser/appcache/appcache_storage_impl.cc
@@ -186,7 +186,8 @@ void AppCacheStorageImpl::DatabaseTask::Schedule() {
       base::Bind(&DatabaseTask::CallRun, this, base::TimeTicks::Now()))) {
     storage_->scheduled_database_tasks_.push_back(this);
   } else {
-    NOTREACHED() << "The database thread is not running.";
+    NOTREACHED() << "Thread for database tasks is not running. "
+                 << "This is not always the DB thread.";
   }
 }