Replace dispatch_async with base::PostTaskWithTraits.

TaskShutdownBehavior::BLOCK_SHUTDOWN is used to make sure that
completion handler is always called on shutdown.

Bug: 853774
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Ief6560419f79055ce9a8d0de47806db5f3f9c9d4
Reviewed-on: https://chromium-review.googlesource.com/1226195Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592517}

ios/web/net/crw_cert_verification_controller.mm

@@ -202,11 +202,10 @@ decideLoadPolicyForRejectedTrustResult:(SecTrustResultType)trustResult
         if (SecTrustEvaluate(trust.get(), &trustResult) != errSecSuccess) {
           trustResult = kSecTrustResultInvalid;
         }
-        // Use GCD API, which is guaranteed to be called during shutdown.
-        // See https://crbug.com/853774 for details.
-        dispatch_async(dispatch_get_main_queue(), ^{
-          completionHandler(trustResult);
-        });
+        TaskTraits traits{WebThread::UI, TaskShutdownBehavior::BLOCK_SHUTDOWN};
+        base::PostTaskWithTraits(FROM_HERE, traits, base::BindOnce(^{
+                                   completionHandler(trustResult);
+                                 }));
       }));
 }
 

ios/web/public/test/web_test_with_web_state.mm

@@ -126,6 +126,7 @@ void WebTestWithWebState::LoadHtml(NSString* html, const GURL& url) {
 
   // Wait until the page is loaded.
   ASSERT_TRUE(WaitUntilConditionOrTimeout(kWaitForPageLoadTimeout, ^{
+    base::RunLoop().RunUntilIdle();
     return web_controller.loadPhase == PAGE_LOADED;
   }));