Try to flush DOMStorage too when ending the browsing session.

content/browser/dom_storage/dom_storage_context_wrapper.cc

Index: content/browser/dom_storage/dom_storage_context_wrapper.cc
diff --git a/content/browser/dom_storage/dom_storage_context_wrapper.cc b/content/browser/dom_storage/dom_storage_context_wrapper.cc
index be4ffb4c0b619f129745bb2b14a1d742ccb5f950..05541ae950535bf55fc1f9b421db287a0e20165d 100644
--- a/content/browser/dom_storage/dom_storage_context_wrapper.cc
+++ b/content/browser/dom_storage/dom_storage_context_wrapper.cc
@@ -248,13 +248,22 @@ void DOMStorageContextWrapper::Shutdown() {
 
 }
 
-void DOMStorageContextWrapper::Flush() {
+std::vector<scoped_refptr<base::SequencedTaskRunner>>
+DOMStorageContextWrapper::Flush() {
   DCHECK(context_.get());
+  std::vector<scoped_refptr<base::SequencedTaskRunner>> flush_runners;
+  if (mojo_state_) {
+    mojo_state_->Flush();
+    // TODO(michaeln): Add mojo impl task runners to flush_runners

[Marijn Kruisselbrink, 2017/04/12 21:55:58]

I'm a bit confused about the implied semantics of the list of task runners being
returned. Is it assumed that all async tasks that are necesary to flush data to
disk have been queued when Flush returns? Because that's going to be hard to
guarantee with the mojo implementation, with its multiple layers of mojo
services on top of each other... 
But anyway, for now the mojo impl would (potentially) involve at least tasks run
on the FILE and DB threads. But most of that is hidden behind mojo interfaces.

+  }
   context_->task_runner()->PostShutdownBlockingTask(
       FROM_HERE, DOMStorageTaskRunner::PRIMARY_SEQUENCE,
       base::Bind(&DOMStorageContextImpl::Flush, context_));
-  if (mojo_state_)
-    mojo_state_->Flush();
+  flush_runners.push_back(context_->task_runner()->GetSequencedTaskRunner(
+      DOMStorageTaskRunner::PRIMARY_SEQUENCE));
+  flush_runners.push_back(context_->task_runner()->GetSequencedTaskRunner(
+      DOMStorageTaskRunner::COMMIT_SEQUENCE));
+  return flush_runners;
 }
 
 void DOMStorageContextWrapper::OpenLocalStorage(