Post a task when invoking CheckDownloadCallback

Currently this could cause an reentrance issues. Destroying a download
will immediately invoke the callback, and the callback will immediately
getting the destroyed DownloadItem.
Posting a task should solve the issue.

BUG=1152451

Change-Id: I8e6f5d64edad2593fa340ab876ebfd858b2a4699
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2561341
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833635}

chrome/browser/safe_browsing/download_protection/check_client_download_request_base.cc

@@ -224,7 +224,9 @@ void CheckClientDownloadRequestBase::FinishRequest(
   if (settings.has_value()) {
     UploadBinary(reason, std::move(settings.value()));
   } else {
-    std::move(callback_).Run(result);
+    // Post a task to avoid reentrance issue. http://crbug.com//1152451.
+    content::GetUIThreadTaskRunner({})->PostTask(
+        FROM_HERE, base::BindOnce(std::move(callback_), result));
   }
 
   UMA_HISTOGRAM_ENUMERATION("SBClientDownload.CheckDownloadStats", reason,

chrome/browser/safe_browsing/download_protection/download_protection_service_unittest.cc

@@ -2414,9 +2414,13 @@ TEST_F(DownloadProtectionServiceTest, TestDownloadItemDestroyed) {
     // notification.
   }
 
+  // Result won't be immediately available as it is being posted.
+  EXPECT_FALSE(has_result_);
+  base::RunLoop().RunUntilIdle();
+
   // When download is destroyed, no need to check for client download request
   // result.
-  EXPECT_TRUE(has_result_);
+  EXPECT_TRUE(IsResult(DownloadCheckResult::UNKNOWN));
   EXPECT_FALSE(HasClientDownloadRequest());
 }
 
@@ -3430,6 +3434,10 @@ TEST_F(DownloadProtectionServiceTest,
       std::move(item),
       base::BindOnce(&DownloadProtectionServiceTest::SyncCheckDoneCallback,
                      base::Unretained(this)));
+  // Result won't be immediately available, wait for the response to
+  // be posted.
+  EXPECT_FALSE(has_result_);
+  base::RunLoop().RunUntilIdle();
   ASSERT_TRUE(IsResult(DownloadCheckResult::WHITELISTED_BY_POLICY));
 }