Implement signin using webview

extensions/browser/extension_function.h

Index: extensions/browser/extension_function.h
diff --git a/extensions/browser/extension_function.h b/extensions/browser/extension_function.h
index f4ffaa273afe5e96007c9a93b7eed5444f87844a..a0b97bc8123b831a663363c0765d46b669033dc3 100644
--- a/extensions/browser/extension_function.h
+++ b/extensions/browser/extension_function.h
@@ -15,6 +15,7 @@
 #include "base/memory/weak_ptr.h"
 #include "base/process/process.h"
 #include "base/sequenced_task_runner_helpers.h"
+#include "base/strings/string_util.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/common/console_message_level.h"
 #include "extensions/browser/extension_function_histogram_value.h"
@@ -218,7 +219,9 @@ class ExtensionFunction
     extension_ = extension;
   }
   const extensions::Extension* extension() const { return extension_.get(); }
-  const std::string& extension_id() const { return extension_->id(); }
+  std::string extension_id() const {
+    return extension_ ? extension_->id() : base::EmptyString();
+  }
 
   void set_request_id(int request_id) { request_id_ = request_id; }
   int request_id() { return request_id_; }