Attach to window, add some editable markers, and add a license file.

go-back-with-backspace/content_script.js

 // Listen for shift-backspace or unmodified backspace and navigate if not in
-// an editable field.
-document.addEventListener('keydown', function(e) {
+// an editable field. We capture the event at the Window to let any handlers
+// or listeners registered on the Document have a chance to handle it first.
+window.addEventListener('keydown', function(e) {
+  // Listening on the Window means the event has no path (see
+  // http://crbug.com/645527), so we'll have to look at the focused (active)
+  // element. This means it will not work properly with shadow DOM.
+  // TODO: Fix behavior with shadow DOM when the above bug is resolved.
   if (e.key === 'Backspace' &&
+      !e.defaultPrevented &&
       !e.altKey &&
       !e.ctrlKey &&
       !e.metaKey &&
-      !isEditable(e.path)) {
+      !isEditable(document.activeElement)) {
     e.shiftKey ? window.history.forward(): window.history.back();
     e.preventDefault();
   }
 });