Add enum that specifies the frame detachment reason.

public/web/WebFrameClient.h

Index: public/web/WebFrameClient.h
diff --git a/public/web/WebFrameClient.h b/public/web/WebFrameClient.h
index 1de7df29d595279f9b1c92d9de8619fe6e361a81..83753e4d56583e146cd3948b0425642c1a3a041d 100644
--- a/public/web/WebFrameClient.h
+++ b/public/web/WebFrameClient.h
@@ -155,6 +155,9 @@ public:
     // See http://html.spec.whatwg.org/#dom-opener.
     virtual void didDisownOpener(WebLocalFrame*) { }
 
+    // Specifies the reason for the detachment.
+    enum class DetachType { Remove, Swap };
+
     // This frame has been detached from the view, but has not been closed yet.
     virtual void frameDetached(WebFrame*) { }