Rewrite PowerSaveBlockerLinux [2/4]

* Removed SelectAPI() which relied on the XDG_CURRENT_DESKTOP environment
  variable. The correct approach is to check the presence of each DBus API
  directly.
* Remove unnecessary |freedesktop_suspend_delegate_|. PowerSaveBlocker::Delegate
  can now hold multiple inhibit cookies, so having multiple delegates is no
  longer needed.

BUG=1013812
TEST=Tested on GNOME and KDE environments
R=blundell

Change-Id: Ia338a12a70e3deed72166e5a980eec64767a7f92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1926852
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718417}

services/device/wake_lock/power_save_blocker/power_save_blocker.h

@@ -61,13 +61,6 @@ class PowerSaveBlocker {
   // };
   scoped_refptr<Delegate> delegate_;
 
-#if defined(USE_X11)
-  // Since display sleep prevention also implies system suspend prevention, for
-  // the Linux FreeDesktop API case, there needs to be a second delegate to
-  // block system suspend when screen saver / display sleep is blocked.
-  scoped_refptr<Delegate> freedesktop_suspend_delegate_;
-#endif
-
   scoped_refptr<base::SequencedTaskRunner> ui_task_runner_;
   scoped_refptr<base::SequencedTaskRunner> blocking_task_runner_;