Revert of Remove calls to MessageLoop::current() in ios. (patchset #1 id:1 of...

Revert of Remove calls to MessageLoop::current() in ios. (patchset #1 id:1 of https://codereview.chromium.org/2079423002/ )

Reason for revert:
This causes crashes during browser startup.

Original issue's description:
> Remove calls to MessageLoop::current() in ios.
>
> Why?
> The fact that there's a MessageLoop on the thread is an
> unnecessary implementation detail. When browser threads
> are migrated to base/task_scheduler, tasks will no longer
> have access to a MessageLoop.
>
> These changes were generated manually.
>
> BUG=616447
> R=droger@chromium.org
>
> Committed: https://crrev.com/c5c127abb5f0d8ba0dd34cfbd6233bbf9209f0f2
> Cr-Commit-Position: refs/heads/master@{#401027}

TBR=sdefresne@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=616447

Review-Url: https://codereview.chromium.org/2326053002
Cr-Commit-Position: refs/heads/master@{#417583}

ios/web/web_thread_impl.cc

@@ -377,7 +377,8 @@ bool WebThread::CurrentlyOn(ID identifier) {
   base::AutoLock lock(globals.lock);
   DCHECK(identifier >= 0 && identifier < ID_COUNT);
   return globals.threads[identifier] &&
-         globals.threads[identifier]->task_runner()->BelongsToCurrentThread();
+         globals.threads[identifier]->message_loop() ==
+             base::MessageLoop::current();
 }
 
 // static