Implement page.setJavaScriptEnabled method

Fixes #462.

docs/api.md

@@ -56,6 +56,7 @@
     + [page.screenshot([options])](#pagescreenshotoptions)
     + [page.setContent(html)](#pagesetcontenthtml)
     + [page.setExtraHTTPHeaders(headers)](#pagesetextrahttpheadersheaders)
+    + [page.setJavaScriptEnabled(enabled)](#pagesetjavascriptenabledenabled)
     + [page.setRequestInterceptionEnabled(value)](#pagesetrequestinterceptionenabledvalue)
     + [page.setUserAgent(userAgent)](#pagesetuseragentuseragent)
     + [page.setViewport(viewport)](#pagesetviewportviewport)
@@ -659,6 +660,11 @@ The extra HTTP headers will be sent with every request the page initiates.
 
 > **NOTE** page.setExtraHTTPHeaders does not guarantee the order of headers in the outgoing requests.
 
+#### page.setJavaScriptEnabled(enabled)
+- `enabled` <[boolean]> Whether or not to enabled JavaScript on the page.
+- returns: <[Promise]>
+
+> **NOTE** changing this value won't affect scripts that have already been run. It will take full effect on the next [navigation](#pagegotourl-options).
 
 #### page.setRequestInterceptionEnabled(value)
 - `value` <[boolean]> Whether to enable request interception.

lib/Page.js

@@ -385,6 +385,13 @@ class Page extends EventEmitter {
     ]);
   }
 
+  /**
+   * @param {boolean} enabled
+   */
+  async setJavaScriptEnabled(enabled) {
+    await this._client.send('Emulation.setScriptExecutionDisabled', { value: !enabled });
+  }
+
   /**
    * @param {?string} mediaType
    * @return {!Promise}

test/test.js

@@ -1696,6 +1696,24 @@ describe('Page', function() {
     }));
   });
 
+  describe('Page.setJavaScriptEnabled', function() {
+    it('should work', SX(async function() {
+      await page.setJavaScriptEnabled(false);
+      await page.goto('data:text/html, <script>var something = "forbidden"</script>');
+      let error = null;
+      try {
+        await page.evaluate('something');
+      } catch (e) {
+        error = e;
+      }
+      expect(error.message).toContain('something is not defined');
+
+      await page.setJavaScriptEnabled(true);
+      await page.goto('data:text/html, <script>var something = "forbidden"</script>');
+      expect(await page.evaluate('something')).toBe('forbidden');
+    }));
+  });
+
   describe('Page.evaluateOnNewDocument', function() {
     it('should evaluate before anything else on the page', SX(async function() {
       await page.evaluateOnNewDocument(function(){