Quick CSS solution like next, also preventing selection via keyboard shortcuts (Ctrl+A):
* {
-webkit-user-select: none;
}
You just did not disable selection to all you elements (some of them still selectable).
I'm did not think that CEF3 currently has any low level solution to prevent text selection. And there is bad idea at all - you pages can contain content editable elements which want selection, so controlling it via CSS looks more flexible.