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This test is flaky on Linux ASAN and Android, sample failure from Android:
```
logging::LogMessage::~LogMessage()
base::internal::WeakReference::Flag::Invalidate()
base::internal::WeakReferenceOwner::~WeakReferenceOwner()
content::SimpleURLLoaderTestHelper::~SimpleURLLoaderTestHelper()
content::(anonymous namespace)::LoadBasicRequestOnIOThread(network::mojom::URLLoaderFactory*, GURL const&)
content::NetworkServiceRestartBrowserTest_BrowserIOSharedFactoryAfterStoragePartitionGone_Test::RunTestOnMainThread()
```

This CL makes sure that |simple_loader_helper.GetCallback()| was
only accessed on UI thread (where |simple_loader_helper| lives).

(I cannot reproduce the flakiness locally, will see if this fixes
the issue.)

Bug: 822585
Change-Id: I74de7dd4708f20dcb2f0145228d6de19ac7f15b3
Reviewed-on: https://chromium-review.googlesource.com/966985
Commit-Queue: Chong Zhang <chongz@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543859}