Attempt to deflake NavigationBrowserTest.StopInOnStart on weblayer

Similar to crrev.com/c/2352268, accessing browser context in weblayer
seems to initialize BackgroundFetchContext, which depends on
ServiceWorkerStorage::GetUserDataForAllRegistrationsByKeyPrefix().
Tests should wait for the completion of
GetUserDataForAllRegistrationsByKeyPrefix() to avoid hitting DCHECK()
in mojo generated bindings.

Bug: 1115172
Change-Id: I5836e962cd35f07ab46e20b64352f8ef8eb9f353
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2368652Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800466}

weblayer/test/weblayer_browser_test.cc

@@ -49,6 +49,9 @@ void WebLayerBrowserTest::PreRunTestOnMainThread() {
       static_cast<BrowserContextImpl*>(browser_context);
   browser_context_impl->profile_impl()->SetDownloadDirectory(
       browser_context->GetPath());
+  // Accessing a browser context may involve storage partition initialization.
+  // Wait for the initialization to be completed.
+  base::RunLoop().RunUntilIdle();
 }
 
 void WebLayerBrowserTest::PostRunTestOnMainThread() {