Add VrShell feature flag and wire it up to VrShell delegate.

content/public/common/content_features.cc

Index: content/public/common/content_features.cc
diff --git a/content/public/common/content_features.cc b/content/public/common/content_features.cc
index 3f969f5d283c970e6094cd0e80d17f46506bbb6a..db5fca8978ee1adb722cb11cdb991e2a2da5fb25 100644
--- a/content/public/common/content_features.cc
+++ b/content/public/common/content_features.cc
@@ -189,6 +189,9 @@ const base::Feature kTokenBinding{"token-binding",
 const base::Feature kTouchpadAndWheelScrollLatching{
     "TouchpadAndWheelScrollLatching", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Enables VR UI.
+const base::Feature kVrShell{"VrShell", base::FEATURE_DISABLED_BY_DEFAULT};
+
 // Enable WebAssembly.
 // http://webassembly.org/
 const base::Feature kWebAssembly{"WebAssembly",