fix a layout test crash due to no browser window

The test opens a link to a pdf, which results in a download and closing the
tab immediately.
As a result, the crash could triggered by potential race between shut down the
content shell and opening the download.

BUG=707599

Review-Url: https://codereview.chromium.org/2842023003
Cr-Commit-Position: refs/heads/master@{#467392}

content/shell/browser/layout_test/layout_test_download_manager_delegate.cc

@@ -35,7 +35,8 @@ LayoutTestDownloadManagerDelegate::~LayoutTestDownloadManagerDelegate(){
 bool LayoutTestDownloadManagerDelegate::ShouldOpenDownload(
       DownloadItem* item,
       const DownloadOpenDelayedCallback& callback) {
-  if (BlinkTestController::Get()->IsMainWindow(item->GetWebContents()) &&
+  if (BlinkTestController::Get() &&
+      BlinkTestController::Get()->IsMainWindow(item->GetWebContents()) &&
       item->GetMimeType() == "text/html") {
     BlinkTestController::Get()->OpenURL(
         net::FilePathToFileURL(item->GetFullPath()));