[CUPS] Fix the width of the printer protocol dropdown menu.

BUG=656807
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2570873003
Cr-Commit-Position: refs/heads/master@{#438639}

chrome/browser/resources/settings/printing_page/cups_add_printer_dialog.html

@@ -95,7 +95,7 @@
         color: var(--google-grey-700);
       }
 
-      paper-dropdown-menu-light,
+      .md-select,
       paper-input {
         --paper-input-container-color: var(--google-grey-500);
         --paper-input-container-input: {