Retain config in WKWebViewConfigurationProviderTest.Purge

On release build, the weak config is deallocated even before the
ASSERT_TRUE.
Add a strong variable to retain it at least in the autoreleasepool.

Note that this test does not test the deletion of the config in the
Purge method as GetWebViewConfiguration returns a copy.

Bug: None
Change-Id: If3e21c3bab53c35859ed1b70c3b207966a02813f
Reviewed-on: https://chromium-review.googlesource.com/579430
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488260}

ios/web/web_state/ui/wk_web_view_configuration_provider_unittest.mm

@@ -122,7 +122,8 @@ TEST_F(WKWebViewConfigurationProviderTest, Purge) {
   __weak id config;
   __weak id router;
   @autoreleasepool {  // Make sure that resulting copy is deallocated.
-    config = GetProvider().GetWebViewConfiguration();
+    id strong_config = GetProvider().GetWebViewConfiguration();
+    config = strong_config;
     router = GetProvider().GetScriptMessageRouter();
     ASSERT_TRUE(config);
     ASSERT_TRUE(router);