Implement Blob support for XMLHttpRequest

Summary:
This PR is a followup to https://github.com/facebook/react-native/pull/11417 and should be merged after that one is merged.

  1. Add support for creating blobs from strings, not just other blobs
  1. Add the `File` constructor which is a superset of `Blob`
  1. Add the `FileReader` API which can be used to read blobs as strings or data url (base64)
  1. Add support for uploading and downloading blobs via `XMLHttpRequest` and `fetch`
  1. Add ability to download local files on Android so you can do `fetch(uri).then(res => res.blob())` to get a blob for a local file (iOS already supported this)

  1. Clone the repo https://github.com/expo/react-native-blob-test
  1. Change the `package.json` and update `react-native` dependency to point to this branch, then run `npm install`
  1. Run the `server.js` file with `node server.js`
  1. Open the `index.common.js` file and replace `localhost` with your computer's IP address
  1. Start the packager with `yarn start` and run the app on your device

If everything went well, all tests should pass, and you should see a screen like this:

![screen shot 2017-06-08 at 7 53 08 pm](https://user-images.githubusercontent.com/1174278/26936407-435bbce2-4c8c-11e7-9ae3-eb104e46961e.png)!

Pull to rerun all tests or tap on specific test to re-run it

  [GENERAL] [FEATURE] [Blob] - Implement blob support for XMLHttpRequest
Closes https://github.com/facebook/react-native/pull/11573

Reviewed By: shergin

Differential Revision: D6082054

Pulled By: hramos

fbshipit-source-id: cc9c174fdefdfaf6e5d9fd7b300120a01a50e8c1

Libraries/WebSocket/RCTWebSocketModule.m

@@ -37,7 +37,7 @@
 @implementation RCTWebSocketModule
 {
   NSMutableDictionary<NSNumber *, RCTSRWebSocket *> *_sockets;
-  NSMutableDictionary<NSNumber *, id> *_contentHandlers;
+  NSMutableDictionary<NSNumber *, id<RCTWebSocketContentHandler>> *_contentHandlers;
 }
 
 RCT_EXPORT_MODULE()
@@ -53,8 +53,9 @@ RCT_EXPORT_MODULE()
            @"websocketClosed"];
 }
 
-- (void)dealloc
+- (void)invalidate
 {
+  _contentHandlers = nil;
   for (RCTSRWebSocket *socket in _sockets.allValues) {
     socket.delegate = nil;
     [socket close];
@@ -135,7 +136,7 @@ RCT_EXPORT_METHOD(close:(nonnull NSNumber *)socketID)
   NSNumber *socketID = [webSocket reactTag];
   id contentHandler = _contentHandlers[socketID];
   if (contentHandler) {
-    message = [contentHandler processMessage:message forSocketID:socketID withType:&type];
+    message = [contentHandler processWebsocketMessage:message forSocketID:socketID withType:&type];
   } else {
     if ([message isKindOfClass:[NSData class]]) {
       type = @"binary";