Add enum that specifies the frame detachment reason.

public/web/WebRemoteFrameClient.h

Index: public/web/WebRemoteFrameClient.h
diff --git a/public/web/WebRemoteFrameClient.h b/public/web/WebRemoteFrameClient.h
index 86df1ca4709461089fb82f537f70a8cf8d2e841d..e11ca39968867b78b89b4e48da19e2ce85dea1bb 100644
--- a/public/web/WebRemoteFrameClient.h
+++ b/public/web/WebRemoteFrameClient.h
@@ -16,6 +16,9 @@ struct WebRect;
 
 class WebRemoteFrameClient {
 public:
+    // Specifies the reason for the detachment.
+    enum class DetachType { Remove, Swap };
+
     // Notify the embedder that it should remove this frame from the frame tree
     // and release any resources associated with it.
     virtual void frameDetached() { }