Move uploadFiles onto ElementHandle (#348)

This patch:
- removes the `page.uploadFile` method
- adds `elementHandle.uploadFile` method.

Motivation: `elementHandle.uploadFile` is rarely used, so it doesn't worth it
to keep it on page.

lib/FrameManager.js

@@ -15,7 +15,6 @@
  */
 
 const fs = require('fs');
-const path = require('path');
 const EventEmitter = require('events');
 const helper = require('./helper');
 const ElementHandle = require('./ElementHandle');
@@ -325,21 +324,6 @@ class Frame {
     return new WaitTask(this, predicateCode, polling, timeout).promise;
   }
 
-  /**
-   * @param {string} selector
-   * @param {!Array<string>} filePaths
-   * @return {!Promise}
-   */
-  async uploadFile(selector, ...filePaths) {
-    let expression = helper.evaluationString(selector => document.querySelector(selector), selector);
-    const {result} = await this._client.send('Runtime.evaluate', { expression });
-    if (!result)
-      return;
-    const objectId = result.objectId;
-    filePaths = filePaths.map(filePath => path.resolve(filePath));
-    return this._client.send('DOM.setFileInputFiles', { objectId, files: filePaths });
-  }
-
   /**
    * @return {!Promise<string>}
    */