Unified: flip mute button activation.

Mute button should have activated color when it's muted. I misunderstood
the spec. See the bug for detail.

TEST=manual
BUG=863514

Change-Id: Ic7d2239c44ed9a00924f3c0c1484570a49f7bbef
Reviewed-on: https://chromium-review.googlesource.com/1194804Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587968}

ash/system/audio/unified_volume_view.cc

@@ -142,7 +142,7 @@ void UnifiedVolumeView::Update(bool by_user) {
   // Indicate that the slider is inactive when it's muted.
   slider()->UpdateState(!is_muted);
 
-  button()->SetToggled(!is_muted);
+  button()->SetToggled(is_muted);
   button()->SetVectorIcon(is_muted ? kUnifiedMenuVolumeMuteIcon
                                    : GetVolumeIconForLevel(level));