WebUI: Prevent text selection on action menu entries.

BUG=709409
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2805283002
Cr-Commit-Position: refs/heads/master@{#462987}

ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.html

@@ -27,6 +27,7 @@
         min-height: 32px;
         padding: 0 24px;
         text-align: start;
+        user-select: none;
         width: 100%;
       }