Files app: Fix flakines on checkContextMenuFocus

Some idea of CL:1670408, wait the focus to be in the desired element.

Test: browser_tests --gtest_filter="*checkContextMenuFocus*"
Bug: 936429
Change-Id: I45ef0be5e6b5c0af004fb881fcf676487f013b4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1679756
Commit-Queue: Noel Gordon <noel@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#673170}