Files app: Fix infinite recursion on search's clear button

Change command default handler for copy/paste/cut/delete to only set
|canExecute| when focusing the element where the default handler was
forced.

This fixes the infinite recursion on <cr-input> and its clear button,
because the handler is forced in the <cr-input> and but the focus
event was triggered in its children <cr-button> (clear button). These
commands should be noop in buttons, these are forced for inputs to be
able to copy/paste/etc the text in the input.

Our error counter doesn't capture this error, so our integration test
can't detect it.

Test: Manually checked.
Bug: 1029213
Fixed: 1029213
Change-Id: I4a9f9d0e05dee8eb18a06359f770498011db0487
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1939999
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719928}

ui/file_manager/file_manager/foreground/js/file_manager_commands.js

@@ -180,7 +180,7 @@ CommandUtil.forceDefaultHandler = (node, commandId) => {
     event.cancelBubble = true;
   });
   node.addEventListener('canExecute', event => {
-    if (event.command.id !== commandId) {
+    if (event.command.id !== commandId || event.target !== node) {
       return;
     }
     event.canExecute = document.queryCommandEnabled(event.command.id);