Add GlobalShortcutListenerLinux

* Add GlobalShortcutListenerLinux to handle global shortcuts on Wayland,
  though the implementation doesn't have any dependencies on Wayland and
  works under X11 as well.  Because of this, the functionality is added
  as a GlobalShortcutListener instead of an Ozone
  PlatformGlobalShortcutListener.  However, we still want the X11
  implementation to take precedence over GlobalShortcutListenerLinux, so
  it's creation is handled in global_shortcut_listener_ozone.cc as a
  fallback if GlobalShortcutListenerOzone creation fails.
* Add a GlobalShortcutsPortal feature flag (disabled by default)
* The org.freedesktop.portal.GlobalShortcuts DBus interface is
  declarative unlike global shortcut registration on other platforms
  (X11, Windows, MacOS) which allows programmatically registering and
  unregistering shortcut keys.  The portal interface takes shortcut IDs
  and descriptions, and key registration is handled by desktop settings.
  This requires adding OnCommandsChanged() to the GlobalShortcutListener
  interface, which takes the whole extension command list.
  Additionally, ExecuteCommand() is added to
  GlobalShortcutListener::Observer as an alternative to
  OnKeyPressed().
* The extension shortcuts web UI page needs to be updated and will
  be done in a followup CL.

Low-Coverage-Reason: TESTS_IN_SEPARATE_CL: Test in https://crrev.com/c/6033575
Change-Id: Iced721a3216a987f979f3f283bc60ffff9788cdc
Bug: 40759171
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5871484
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1385140}