<webview>: Make contentWindow available prior to attachment (on display:none).

extensions/common/guest_view/guest_view_constants.cc

Index: extensions/common/guest_view/guest_view_constants.cc
diff --git a/extensions/common/guest_view/guest_view_constants.cc b/extensions/common/guest_view/guest_view_constants.cc
index 3ce10726c4b40dea27b99a44132aac82846c383a..c35c59f2b097a3c38c4b8e1c8a1b0df2d6b046a8 100644
--- a/extensions/common/guest_view/guest_view_constants.cc
+++ b/extensions/common/guest_view/guest_view_constants.cc
@@ -19,6 +19,8 @@ const char kElementHeight[] = "elementHeight";
 const char kEventResize[] = "guestViewInternal.onResize";
 
 // Parameters/properties on events.
+const char kContentWindowID[] = "contentWindowId";
+const char kID[] = "id";
 const char kIsTopLevel[] = "isTopLevel";
 const char kNewWidth[] = "newWidth";
 const char kNewHeight[] = "newHeight";