Add Task Manager to ChromeOS shelf context menu

Currently there is no way to open TaskManager using mouse/touch
if there is no browser window, even though you still can open it
via shortcut.

This CL adds a new menu item 'Task manager' to the shelf context menu.
Selecting this item opens the Chrome OS Task Manager.

Bug: None
Change-Id: Icd3afd462dd8544f9be5952972740424cddb6993
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7850774
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Michael Wojcicka <mwoj@google.com>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Reviewed-by: Dibyajyoti Pal <dibyapal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1648188}