[Files app] Rename test helper

Rename test helper to follow the convention of other helper code that
traverses into the shadow DOM.

Bug: 907380
Change-Id: I7f6ee251a08995aa6b4052f7a87f4d6e3230414b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2003154
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732240}

ui/file_manager/file_manager/background/js/runtime_loaded_test_util.js

@@ -299,7 +299,7 @@ test.util.sync.getActiveElement = (contentWindow, opt_styleNames) => {
  *     values, hidden attribute, and style names and values. If there is no
  *     active element, returns null.
  */
-test.util.sync.findActiveElement = (contentWindow, opt_styleNames) => {
+test.util.sync.deepGetActiveElement = (contentWindow, opt_styleNames) => {
   if (!contentWindow.document || !contentWindow.document.activeElement) {
     return null;
   }