Use views::style for buttons, bootstrap ash_typography to do so.

ash/system/session/logout_button_tray.cc

Index: ash/system/session/logout_button_tray.cc
diff --git a/ash/system/session/logout_button_tray.cc b/ash/system/session/logout_button_tray.cc
index fb0cf211a8ee030d9575e3fc9fe1722a9e06b6be..c11623682ab28f8acd9876325331e58f41d69a37 100644
--- a/ash/system/session/logout_button_tray.cc
+++ b/ash/system/session/logout_button_tray.cc
@@ -7,6 +7,7 @@
 #include <memory>
 #include <utility>
 
+#include "ash/ash_typography.h"
 #include "ash/public/cpp/shelf_types.h"
 #include "ash/resources/grit/ash_resources.h"
 #include "ash/resources/vector_icons/vector_icons.h"
@@ -38,14 +39,10 @@ LogoutButtonTray::LogoutButtonTray(WmShelf* wm_shelf)
       button_(nullptr),
       login_status_(LoginStatus::NOT_LOGGED_IN),
       show_logout_button_in_tray_(false) {
-  views::MdTextButton* button =
-      views::MdTextButton::Create(this, base::string16());
+  views::MdTextButton* button = views::MdTextButton::Create(
+      this, base::string16(), CONTEXT_LAUNCHER_BUTTON);
   button->SetProminent(true);
   button->SetBgColorOverride(gfx::kGoogleRed700);
-  // Base font size + 2 = 14.
-  // TODO(estade): should this 2 be shared with other tray views? See
-  // crbug.com/623987
-  button->AdjustFontSize(2);
   button_ = button;
 
   // Since LogoutButtonTray has a red background and it is distinguished