Resolve paths against CWD in page.uploadFile() method

This patch:
- teaches page.uploadFile() to resolve given file paths against
  current working directory. This aligns paths handling with all the
  other methods
- moves page.uploadFile() under Frame
- changes test to use relative path for file upload

lib/FrameManager.js

@@ -15,6 +15,7 @@
  */
 
 let fs = require('fs');
+let path = require('path');
 let EventEmitter = require('events');
 let helper = require('./helper');
 
@@ -323,6 +324,21 @@ 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 });
+  }
+
   /**
    * @template T
    * @param {string} selector

lib/Page.js

@@ -590,12 +590,7 @@ class Page extends EventEmitter {
    * @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;
-    return this._client.send('DOM.setFileInputFiles', { objectId, files: filePaths });
+    return this.mainFrame().uploadFile(selector, ...filePaths);
   }
 
   /**