Add a new metric for how many times permissions prompts are shown.

chrome/browser/download/download_permission_request.cc

Index: chrome/browser/download/download_permission_request.cc
diff --git a/chrome/browser/download/download_permission_request.cc b/chrome/browser/download/download_permission_request.cc
index a8a35ad0383afb256ba6f70fcc64dc228c47e51b..7e88c78ce95f1fd300dbda25d07034cf7056bbda 100644
--- a/chrome/browser/download/download_permission_request.cc
+++ b/chrome/browser/download/download_permission_request.cc
@@ -59,3 +59,8 @@ void DownloadPermissionRequest::Cancelled() {
 void DownloadPermissionRequest::RequestFinished() {
   delete this;
 }
+
+PermissionBubbleType DownloadPermissionRequest::GetPermissionBubbleType()
+    const {
+  return PermissionBubbleType::DOWNLOAD;
+}