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

chrome/browser/download/chrome_download_manager_delegate.h

Index: chrome/browser/download/chrome_download_manager_delegate.h
diff --git a/chrome/browser/download/chrome_download_manager_delegate.h b/chrome/browser/download/chrome_download_manager_delegate.h
index a03a227c757472cb4748b707f712c5c0c57ea1bc..3573cd7ff0b5baa1a8fac4dbfc00808ea01420e4 100644
--- a/chrome/browser/download/chrome_download_manager_delegate.h
+++ b/chrome/browser/download/chrome_download_manager_delegate.h
@@ -121,6 +121,7 @@ class ChromeDownloadManagerDelegate
                         const CheckDownloadUrlCallback& callback) override;
   void GetFileMimeType(const base::FilePath& path,
                        const GetFileMimeTypeCallback& callback) override;
+  void RecordDriveByPDFDownload() const override;
 
   // So that test classes that inherit from this for override purposes
   // can call back into the DownloadManager.