Add context menu tests for Myfiles,  Downloads, Linux and Play files

Move expandTreeItem helper function from folder_shortcuts.js to
background.js to be able to reuse on other tests.

Bug: 925516
Change-Id: I95f425c5a6fbdd5afa424cbfd1d08b1d87cc59db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504658
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638048}
4 files changed