Added common layout framework for system menu rows.

ash/common/system/tray/tray_constants.h

Index: ash/common/system/tray/tray_constants.h
diff --git a/ash/common/system/tray/tray_constants.h b/ash/common/system/tray/tray_constants.h
index cd1c8e851c36c282c8f1780fca5bcd2f3c831f19..61d9e180a5db638e24ca8cd227f264c5ee3a7141 100644
--- a/ash/common/system/tray/tray_constants.h
+++ b/ash/common/system/tray/tray_constants.h
@@ -114,6 +114,18 @@ enum TrayConstant {
   // The height of the rows in the system tray menu.
   TRAY_POPUP_ITEM_HEIGHT,
 
+  // The left inset for all tray system menu rows.
+  TRAY_POPUP_ITEM_LEFT_INSET,
+
+  // The right inset for all tray system menu rows.
+  TRAY_POPUP_ITEM_RIGHT_INSET,
+
+  // The minimum default width for the left container of the system menu rows.
+  TRAY_POPUP_ITEM_MIN_START_WIDTH,
+
+  // The minimum default width for the right container of the system menu rows.
+  TRAY_POPUP_ITEM_MIN_END_WIDTH,
+
   // The width and height of the virtual keyboard button in the status tray
   // area. For non-MD, adjustments are made to the button dimensions based on
   // the shelf orientation, so this constant does not specify the true