Move impl of identity.GetAccounts() extension API to its own file

chrome/browser/extensions/api/identity/identity_get_accounts_function.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_EXTENSIONS_API_IDENTITY_IDENTITY_GET_ACCOUNTS_FUNCTION_H_
+#define CHROME_BROWSER_EXTENSIONS_API_IDENTITY_IDENTITY_GET_ACCOUNTS_FUNCTION_H_
+
+#include "chrome/browser/extensions/chrome_extension_function.h"
+#include "extensions/browser/extension_function_histogram_value.h"
+
+namespace extensions {
+
+class IdentityGetAccountsFunction : public ChromeUIThreadExtensionFunction {
+ public:
+  DECLARE_EXTENSION_FUNCTION("identity.getAccounts",
+                             IDENTITY_GETACCOUNTS);
+
+  IdentityGetAccountsFunction();
+
+ private:
+  ~IdentityGetAccountsFunction() override;
+
+  // UIThreadExtensionFunction implementation.
+  ExtensionFunction::ResponseAction Run() override;
+};
+
+}  // namespace extensions
+
+#endif  // CHROME_BROWSER_EXTENSIONS_API_IDENTITY_IDENTITY_GET_ACCOUNTS_FUNCTION_H_