PDF: Set a referrer when performing a SaveAs operation.

Currently, the plugin's frame's url is
chrome-extension://[pdf_extension_id]/index-material.html
and that gets sanitized to nothing. Instead, just use the document's
URL, like in r327641.

BUG=563137

Review URL: https://codereview.chromium.org/1510993002

Cr-Commit-Position: refs/heads/master@{#364498}

components/pdf/renderer/pepper_pdf_host.cc

@@ -153,11 +153,10 @@ int32_t PepperPDFHost::OnHostMsgSaveAs(
     return PP_ERROR_FAILED;
   GURL url = instance->GetPluginURL();
   content::RenderView* render_view = instance->GetRenderView();
-  blink::WebLocalFrame* frame =
-      render_view->GetWebView()->mainFrame()->toWebLocalFrame();
-  content::Referrer referrer = content::Referrer::SanitizeForRequest(
-      url, content::Referrer(frame->document().url(),
-                             frame->document().referrerPolicy()));
+  content::Referrer referrer;
+  referrer.url = url;
+  referrer.policy = blink::WebReferrerPolicyDefault;
+  referrer = content::Referrer::SanitizeForRequest(url, referrer);
   render_view->Send(
       new PDFHostMsg_PDFSaveURLAs(render_view->GetRoutingID(), url, referrer));
   return PP_OK;