Introduce new flag to control usage of MD in secondary (not top-chrome)

ui/base/ui_base_switches.cc

Index: ui/base/ui_base_switches.cc
diff --git a/ui/base/ui_base_switches.cc b/ui/base/ui_base_switches.cc
index 7ec3c6bf649407b9872d4f12974a9601f40c8a16..4fbeb252d4513b2d19d160d0521adcffc7d78299 100644
--- a/ui/base/ui_base_switches.cc
+++ b/ui/base/ui_base_switches.cc
@@ -80,6 +80,10 @@ const char kTopChromeMDMaterialHybrid[] = "material-hybrid";
 
 // Classic, non-material, mode for the |kTopChromeMD| switch.
 const char kTopChromeMDNonMaterial[] = "non-material";
+
+// Applies the material design mode passed via --top-chrome-md to elements
+// throughout Chrome (not just top Chrome).
+const char kExtendMdToSecondaryUi[] = "secondary-ui-md";
 #endif  // defined(ENABLE_TOPCHROME_MD)
 
 }  // namespace switches