Use matching RVH/RFH in DownloadUrlParameters for PDF plugin placeholder.

BUG=881497
TEST=See bug for repro steps.

Change-Id: I392788d31e638bc79bb8d35e0451c79af6ca3a26
Reviewed-on: https://chromium-review.googlesource.com/1212145Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589559}

chrome/browser/plugins/pdf_plugin_placeholder_observer.cc

@@ -94,8 +94,8 @@ void PDFPluginPlaceholderObserver::OnOpenPDF(
         })");
   std::unique_ptr<download::DownloadUrlParameters> params =
       std::make_unique<download::DownloadUrlParameters>(
-          url, web_contents()->GetRenderViewHost()->GetProcess()->GetID(),
-          web_contents()->GetRenderViewHost()->GetRoutingID(),
+          url, render_frame_host->GetRenderViewHost()->GetProcess()->GetID(),
+          render_frame_host->GetRenderViewHost()->GetRoutingID(),
           render_frame_host->GetRoutingID(),
           storage_partition->GetURLRequestContext(), traffic_annotation);
   params->set_referrer(referrer.url);