Print Preview: Clear callbacks map when JS disallowed

In normal print preview dialogs, the handler is destroyed with the web
contents, but for the special case of chrome://print this does not
occur, and old preview requests should be cleared in
OnJavascriptDisallowed().

Bug: 853672
Change-Id: I5681680dc64fe020f389c6893d992694e8fcdb95
Reviewed-on: https://chromium-review.googlesource.com/1104877Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568121}

chrome/browser/ui/webui/print_preview/print_preview_handler.cc

@@ -589,6 +589,7 @@ void PrintPreviewHandler::OnJavascriptDisallowed() {
   // Normally the handler and print preview will be destroyed together, but
   // this is necessary for refresh or navigation from the chrome://print page.
   weak_factory_.InvalidateWeakPtrs();
+  preview_callbacks_.clear();
   UnregisterForGaiaCookieChanges();
 }