PDF: Add UMA to track embedded PDF triggered drive-by downloads.

content/browser/download/download_manager_impl.cc

Index: content/browser/download/download_manager_impl.cc
diff --git a/content/browser/download/download_manager_impl.cc b/content/browser/download/download_manager_impl.cc
index 44ac7f581afaafa94fd42f39bbb25a546d2f11fb..7b233ee51f2e7df09cf7b8e50e6326ebfe7f15df 100644
--- a/content/browser/download/download_manager_impl.cc
+++ b/content/browser/download/download_manager_impl.cc
@@ -174,6 +174,8 @@ class DownloadItemFactoryImpl : public DownloadItemFactory {
   }
 };
 
+constexpr char kPDFMimeType[] = "application/pdf";
+
 }  // namespace
 
 DownloadManagerImpl::DownloadManagerImpl(net::NetLog* net_log,
@@ -203,6 +205,10 @@ DownloadItemImpl* DownloadManagerImpl::CreateActiveItem(
       item_factory_->CreateActiveItem(this, id, info, net_log);
   downloads_[id] = base::WrapUnique(download);
   downloads_by_guid_[download->GetGuid()] = download;
+
+  if (info.mime_type == kPDFMimeType && !info.has_user_gesture)
+    delegate_->RecordDriveByPDFDownload();
+
   return download;
 }