dom_storage: speculative crash fix

A crash seems to indicate LocalStorageContextMojo::PerformStorageCleanup()
is being called with |this| set to null. It seems the only way that could
happen is if DOMStorageContextWrapper::PerformLocalStorageCleanup() is called
after Shutdown() (Shutdown() sets mojo_state_ to null).

This is a speculative fix, and I could certainly be wrong. Please suggest
alternatives if you think I'm reading it wrong or what I've outlined isn't
possible.

BUG=921067
TEST=none

Change-Id: I0eb7482bf37369e4b61b05d34526c679fa71d416
Reviewed-on: https://chromium-review.googlesource.com/c/1407117
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622952}

content/browser/dom_storage/dom_storage_context_wrapper.cc

@@ -275,6 +275,11 @@ void DOMStorageContextWrapper::PerformLocalStorageCleanup(
     base::OnceClosure callback) {
   DCHECK(context_.get());
   DCHECK(callback);
+  if (!mojo_state_) {
+    // Shutdown() has been called.
+    std::move(callback).Run();
+    return;
+  }
   mojo_task_runner_->PostTask(
       FROM_HERE,
       base::BindOnce(