Disable back button when stack is empty

chrome/browser/android/vr_shell/color_scheme.cc

Index: chrome/browser/android/vr_shell/color_scheme.cc
diff --git a/chrome/browser/android/vr_shell/color_scheme.cc b/chrome/browser/android/vr_shell/color_scheme.cc
index f43273e99d825d6b155a8919628d08603daca4c7..49432770da2703ff5c8e26b695a93b8ad63e2b95 100644
--- a/chrome/browser/android/vr_shell/color_scheme.cc
+++ b/chrome/browser/android/vr_shell/color_scheme.cc
@@ -32,6 +32,7 @@ void InitializeColorSchemes() {
   normal_scheme.secure = gfx::kGoogleGreen700;
   normal_scheme.insecure = gfx::kGoogleRed700;
   normal_scheme.warning = normal_scheme.deemphasized;
+  normal_scheme.disabled = 0x33333333;
 
   kColorSchemes[ColorScheme::kModeFullscreen] =
       kColorSchemes[ColorScheme::kModeNormal];
@@ -57,6 +58,7 @@ void InitializeColorSchemes() {
   incognito_scheme.secure = incognito_scheme.foreground;
   incognito_scheme.insecure = incognito_scheme.foreground;
   incognito_scheme.warning = incognito_scheme.foreground;
+  incognito_scheme.disabled = 0x33E6E6E6;
 
   initialized = true;
 }