Make normal MD text buttons *on infobars* white.

ui/views/controls/button/md_text_button.h

Index: ui/views/controls/button/md_text_button.h
diff --git a/ui/views/controls/button/md_text_button.h b/ui/views/controls/button/md_text_button.h
index 687de5e43f5c9ba783c4f65c247c21f07ddbfe2e..eb6b6fee07dce8a360740be9d6f03150283a973e 100644
--- a/ui/views/controls/button/md_text_button.h
+++ b/ui/views/controls/button/md_text_button.h
@@ -7,6 +7,7 @@
 
 #include <memory>
 
+#include "base/optional.h"
 #include "ui/views/controls/button/label_button.h"
 
 namespace views {
@@ -38,6 +39,7 @@ class VIEWS_EXPORT MdTextButton : public LabelButton {
                                SkAlpha alpha);
 
   void SetCallToAction(bool cta);
+  void set_bg_color_override(SkColor color) { bg_color_override_ = color; }
 
   // LabelButton:
   void Layout() override;
@@ -65,6 +67,9 @@ class VIEWS_EXPORT MdTextButton : public LabelButton {
   // True if this button uses call-to-action styling.
   bool is_cta_;
 
+  // When set, this provides the background color.
+  base::Optional<SkColor> bg_color_override_;
+
   DISALLOW_COPY_AND_ASSIGN(MdTextButton);
 };