Add guard to ErrorUtils mock

Summary:
This method is defined in the implementation but is missing from the mock.

public

Reviewed By: cpojer

Differential Revision: D2875173

fb-gh-sync-id: 6544c34a3d707ff9cecacc0608ab8320b950bfb7

Libraries/Utilities/__mocks__/ErrorUtils.js

@@ -15,6 +15,7 @@ function reportError(error) {
 var ErrorUtils = {
   apply: jest.genMockFunction().mockImplementation(execute),
   applyWithGuard: jest.genMockFunction().mockImplementation(execute),
+  guard: jest.genMockFunction().mockImplementation(callback => callback),
   inGuard: jest.genMockFunction().mockReturnValue(true),
   reportError: jest.genMockFunction().mockImplementation(reportError),
   setGlobalHandler: jest.genMockFunction(),