Introduce page.emulate convenience wrapper (#231)

This patch:
- introduces Page.emulate convenience wrapper
- adds a screenshot-fullPage.js example

examples/pdf.js

@@ -25,7 +25,7 @@ await page.goto('https://news.ycombinator.com', {waitUntil: 'networkidle'});
 // @see https://bugs.chromium.org/p/chromium/issues/detail?id=753118
 await page.pdf({
   path: 'hn.pdf',
-  format: 'A4'
+  format: 'letter'
 });
 
 browser.close();

examples/screenshot-fullpage.js

@@ -0,0 +1,26 @@
+/**
+ * Copyright 2017 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+const {Browser} = require('puppeteer');
+const devices = require('puppeteer/DeviceDescriptors');
+const browser = new Browser();
+
+browser.newPage().then(async page => {
+  await page.emulate(devices['iPhone 6']);
+  await page.navigate('https://www.nytimes.com/');
+  await page.screenshot({path: 'fp_page.png', fullPage: true});
+  browser.close();
+});