linux and chromeos: Turn on -Wunused-const-variable.

ash/launcher/launcher_button.cc

Index: ash/launcher/launcher_button.cc
diff --git a/ash/launcher/launcher_button.cc b/ash/launcher/launcher_button.cc
index 0678d159d9e130ffa84925c1b19b814032c3cb79..9f0b32ea3b7b170a6b40fb87ba3096c2f0c20188 100644
--- a/ash/launcher/launcher_button.cc
+++ b/ash/launcher/launcher_button.cc
@@ -29,7 +29,6 @@ namespace {
 // Size of the bar. This is along the opposite axis of the shelf. For example,
 // if the shelf is aligned horizontally then this is the height of the bar.
 const int kBarSize = 3;
-const int kBarSpacing = 5;
 const int kIconSize = 32;
 const int kHopSpacing = 2;
 const int kIconPad = 8;