Introduce Page.saveScreenshot

Page.saveScreenshot uses filepath to infer screenshot type (either
PNG of JPG) and saves the screenshot to file system.

lib/Page.js

@@ -17,6 +17,7 @@
 var fs = require('fs');
 var EventEmitter = require('events');
 var helpers = require('./helpers');
+var mime = require('mime');
 
 class Page extends EventEmitter {
     /**
@@ -335,6 +336,24 @@ class Page extends EventEmitter {
         return new Buffer(result.data, 'base64');
     }
 
+    /**
+     * @param {string} filePath
+     * @param {?{x: number, y: number, width: number, height: number}} clipRect
+     * @return {!Promise}
+     */
+    async saveScreenshot(filePath, clipRect) {
+        var mimeType = mime.lookup(filePath);
+        var screenshotType = null;
+        if (mimeType === 'image/png')
+            screenshotType = Page.ScreenshotTypes.PNG;
+        else if (mimeType === 'image/jpeg')
+            screenshotType = Page.ScreenshotTypes.JPG;
+        if (!screenshotType)
+            throw new Error(`Cannot render to file ${fileName} - unsupported mimeType ${mimeType}`);
+        var buffer = await this.screenshot(screenshotType, clipRect);
+        fs.writeFileSync(filePath, buffer);
+    }
+
     /**
      * @return {!Promise<string>}
      */