<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 f478c64a33ee3c7bd883b7d56ef66da2d87335a1..94af1b5eceac77315e3056903ea1ee528696aa23 100644
--- a/extensions/common/guest_view/guest_view_constants.cc
+++ b/extensions/common/guest_view/guest_view_constants.cc
@@ -20,6 +20,8 @@ const char kElementSizeIsLogical[] = "elementSizeIsLogical";
 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";