Add support for ontimeout and onerror handler when using XMLHttpRequest for Android and iOS

Summary:Currently React-Native does not have `ontimeout` and `onerror` handlers for [XMLHttpRequest](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest). This is an extension to [No timeout on XMLHttpRequest](https://github.com/facebook/react-native/issues/4648).

With addition to two handlers, both Android and iOS can now handle `ontimeout` if request times out and `onerror` when there is general network error.

**Test plan**

Code has been tested on both Android and iOS with [Charles](https://www.charlesproxy.com/) by setting a breakpoint on the request which fires `ontimeout` when the request waits beyond `timeout` time and `onerror` when there is network error.

**Usage**

JavaScript -

```
var request = new XMLHttpRequest();

function onLoad() {
    console.log(request.status);
};

function onTimeout() {
    console.log('Timeout');
};

function onError() {
    console.log('General network error');
};

request.onload = onLoad;
request.ontimeout = onTimeout;
request.onerr
Closes https://github.com/facebook/react-native/pull/6841

Differential Revision: D3178859

Pulled By: lexs

fb-gh-sync-id: 30674570653e92ab5f7e74bd925dd5640fc862b6
fbshipit-source-id: 30674570653e92ab5f7e74bd925dd5640fc862b6

Libraries/Network/__tests__/XMLHttpRequestBase-test.js

@@ -0,0 +1,48 @@
+'use strict';
+
+jest
+	.autoMockOff()
+	.dontMock('XMLHttpRequestBase');
+
+const XMLHttpRequestBase = require('XMLHttpRequestBase');
+
+describe('XMLHttpRequestBase', function(){
+	var xhr;
+
+	beforeEach(() => {
+		xhr = new XMLHttpRequestBase();
+		xhr.ontimeout = jest.fn();
+		xhr.onerror = jest.fn();
+		xhr.onload = jest.fn();
+		xhr.didCreateRequest(1);
+	});
+
+	afterEach(() => {
+		xhr = null;
+	});
+
+	it('should call ontimeout function when the request times out', function(){
+		xhr._didCompleteResponse(1, 'Timeout', true);
+
+		expect(xhr.ontimeout).toBeCalledWith(null);
+		expect(xhr.onerror).not.toBeCalled();
+		expect(xhr.onload).not.toBeCalled();
+	});
+
+	it('should call onerror function when the request times out', function(){
+		xhr._didCompleteResponse(1, 'Generic error');
+
+		expect(xhr.onerror).toBeCalledWith(null);
+		expect(xhr.ontimeout).not.toBeCalled();
+		expect(xhr.onload).not.toBeCalled();
+	});
+
+	it('should call onload function when there is no error', function(){
+		xhr._didCompleteResponse(1, null);
+
+		expect(xhr.onload).toBeCalledWith(null);
+		expect(xhr.onerror).not.toBeCalled();
+		expect(xhr.ontimeout).not.toBeCalled();
+	});
+
+});