Add CanCloseDialog to WebDialogDelegate to allow blocking closing of dialog if needed.

ui/web_dialogs/web_dialog_delegate.h

Index: ui/web_dialogs/web_dialog_delegate.h
diff --git a/ui/web_dialogs/web_dialog_delegate.h b/ui/web_dialogs/web_dialog_delegate.h
index 8416c1f15e051da190074b25fe1cd8f24c9bd872..2bd772f018e2e49f4424aa20bc9498f86ce1789c 100644
--- a/ui/web_dialogs/web_dialog_delegate.h
+++ b/ui/web_dialogs/web_dialog_delegate.h
@@ -63,6 +63,12 @@ class WEB_DIALOGS_EXPORT WebDialogDelegate {
   // Gets the JSON string input to use when showing the dialog.
   virtual std::string GetDialogArgs() const = 0;
 
+  // Returns true to signal that the dialog can be closed. Specialized
+  // WebDialogDelegate subclasses can override this default behavior to allow
+  // the close to be blocked until the user corrects mistakes, accepts an
+  // agreement, etc.
+  virtual bool CanCloseDialog() const;
+
   // A callback to notify the delegate that |source|'s loading state has
   // changed.
   virtual void OnLoadingStateChanged(content::WebContents* source) {}