Share a common alpha value used for displaying disabled state in Harmony

ui/views/controls/button/md_text_button.cc

Index: ui/views/controls/button/md_text_button.cc
diff --git a/ui/views/controls/button/md_text_button.cc b/ui/views/controls/button/md_text_button.cc
index 3e582db3ca6957318412b8a37b1d2f823aad429d..327c347b55c83102a79e02bfaf0d71be9f2c9f6d 100644
--- a/ui/views/controls/button/md_text_button.cc
+++ b/ui/views/controls/button/md_text_button.cc
@@ -7,6 +7,7 @@
 #include "base/i18n/case_conversion.h"
 #include "ui/base/material_design/material_design_controller.h"
 #include "ui/gfx/canvas.h"
+#include "ui/gfx/color_palette.h"
 #include "ui/gfx/color_utils.h"
 #include "ui/native_theme/native_theme.h"
 #include "ui/views/animation/flood_fill_ink_drop_ripple.h"
@@ -307,7 +308,8 @@ void MdTextButton::UpdateColors() {
     bg_color = theme->GetSystemColor(
         ui::NativeTheme::kColorId_CallToActionColor);
     if (state() == STATE_DISABLED)
-      bg_color = color_utils::BlendTowardOppositeLuma(bg_color, 0x61);
+      bg_color = color_utils::BlendTowardOppositeLuma(
+          bg_color, gfx::kDisabledControlAlpha);
   } else if (is_default()) {
     bg_color = color_utils::BlendTowardOppositeLuma(text_color, 0xD8);
   }