Make the UI an observer of downloads.

content/browser/android/download_controller_android_impl.cc

Index: content/browser/android/download_controller_android_impl.cc
diff --git a/content/browser/android/download_controller_android_impl.cc b/content/browser/android/download_controller_android_impl.cc
index 2bd324ab2d60f460b7ba2bf7d77eb2698ae09691..db47058548f5cd92ce5dfe17047fb40b10356c39 100644
--- a/content/browser/android/download_controller_android_impl.cc
+++ b/content/browser/android/download_controller_android_impl.cc
@@ -235,16 +235,18 @@ void DownloadControllerAndroidImpl::StartAndroidDownload(
 }
 
 void DownloadControllerAndroidImpl::OnPostDownloadStarted(
-    WebContents* web_contents,
     DownloadItem* download_item) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  if (!download_item->GetWebContents())
+    return;
+
   JNIEnv* env = AttachCurrentThread();
 
   // Register for updates to the DownloadItem.
   download_item->AddObserver(this);
 
   ScopedJavaLocalRef<jobject> view =
-      GetContentViewCoreFromWebContents(web_contents);
+      GetContentViewCoreFromWebContents(download_item->GetWebContents());
   if(view.is_null()) {
     // The view went away. Can't proceed.
     return;