Implement Body interface for both Request and Response

This patch partially implements Fetch API's [Body](https://developer.mozilla.org/en-US/docs/Web/API/Body)
interface for both Request and Response.

Fixes #26.

lib/NetworkManager.js

@@ -94,7 +94,7 @@ class NetworkManager extends EventEmitter {
   _onRequestWillBeSent(event) {
     if (event.redirectResponse) {
       let request = this._idToRequest.get(event.requestId);
-      let response = new Response(request, event.redirectResponse);
+      let response = new Response(request, event.redirectResponse, this._getResponseBody.bind(this, event.requestId));
       request._response = response;
       this.emit(NetworkManager.Events.Response, response);
       this.emit(NetworkManager.Events.RequestFinished, request);
@@ -109,7 +109,7 @@ class NetworkManager extends EventEmitter {
    */
   _onResponseReceived(event) {
     let request = this._idToRequest.get(event.requestId) || null;
-    let response = new Response(request, event.response);
+    let response = new Response(request, event.response, this._getResponseBody.bind(this, event.requestId));
     request._response = response;
     this.emit(NetworkManager.Events.Response, response);
   }
@@ -131,6 +131,15 @@ class NetworkManager extends EventEmitter {
     this._idToRequest.delete(event.requestId);
     this.emit(NetworkManager.Events.RequestFailed, request);
   }
+
+  /**
+   * @param {string} requestId
+   * @return {!Promise<!Buffer>}
+   */
+  async _getResponseBody(requestId) {
+    let response = await this._client.send('Network.getResponseBody', {requestId});
+    return Buffer.from(response.body, response.base64Encoded ? 'base64' : 'utf8');
+  }
 }
 
 class Headers {
@@ -218,11 +227,63 @@ class Headers {
   }
 }
 
-class Request {
+class Body {
+  /**
+   * @param {function():!Promise<!Buffer>} contentCallback
+   */
+  constructor(contentCallback) {
+    this._contentCallback = contentCallback;
+    /** @type {?Promise<!Buffer>} */
+    this._contentPromise = null;
+  }
+
+  /**
+   * @return {!Promise<!Buffer>}
+   */
+  buffer() {
+    if (!this._contentPromise)
+      this._contentPromise = this._contentCallback();
+    return this._contentPromise;
+  }
+
+  /**
+   * @return {boolean}
+   */
+  get bodyUsed() {
+    return !!this._contentPromise;
+  }
+
+  /**
+   * @return {!Promise<string>}
+   */
+  async text() {
+    let content = await this.buffer();
+    return content.toString('utf8');
+  }
+
+  /**
+   * @return {!Promise<!Object>}
+   */
+  async json() {
+    let content = await this.text();
+    return JSON.parse(content);
+  }
+
+  /**
+   * @return {!Promise<!ArrayBuffer>}
+   */
+  async arrayBuffer() {
+    let content = await this.buffer();
+    return content.buffer;
+  }
+}
+
+class Request extends Body {
   /**
    * @param {!Object} payload
    */
   constructor(payload) {
+    super(() => Promise.resolve(payload.postData || ''));
     this._response = null;
     this.url = payload.url;
     this.method = payload.method;
@@ -238,12 +299,14 @@ class Request {
   }
 }
 
-class Response {
+class Response extends Body {
   /**
    * @param {?Request} request
    * @param {!Object} payload
+   * @param {function():!Promise<!Buffer>} contentCallback
    */
-  constructor(request, payload) {
+  constructor(request, payload, contentCallback) {
+    super(contentCallback);
     this._request = request;
     this.headers = Headers.fromPayload(payload.headers);
     this.ok = payload.status >= 200 && payload.status <= 299;