fix(tracing): start without options (#4388)

docs/api.md

@@ -180,7 +180,7 @@
 - [class: Touchscreen](#class-touchscreen)
   * [touchscreen.tap(x, y)](#touchscreentapx-y)
 - [class: Tracing](#class-tracing)
-  * [tracing.start(options)](#tracingstartoptions)
+  * [tracing.start([options])](#tracingstartoptions)
   * [tracing.stop()](#tracingstop)
 - [class: Dialog](#class-dialog)
   * [dialog.accept([promptText])](#dialogacceptprompttext)
@@ -2319,7 +2319,7 @@ await page.goto('https://www.google.com');
 await page.tracing.stop();
 ```
 
-#### tracing.start(options)
+#### tracing.start([options])
 - `options` <[Object]>
   - `path` <[string]> A path to write the trace file to.
   - `screenshots` <[boolean]> captures screenshots in the trace.

lib/Tracing.js

@@ -31,9 +31,9 @@ class Tracing {
   }
 
   /**
-   * @param {!{path: string, screenshots?: boolean, categories?: !Array<string>}} options
+   * @param {!{path?: string, screenshots?: boolean, categories?: !Array<string>}} options
    */
-  async start(options) {
+  async start(options = {}) {
     assert(!this._recording, 'Cannot start recording trace while already recording trace.');
 
     const defaultCategories = [
@@ -75,7 +75,7 @@ class Tracing {
 
   /**
    * @param {string} handle
-   * @param {string} path
+   * @param {?string} path
    */
   async _readStream(handle, path) {
     let eof = false;

test/tracing.spec.js

@@ -66,6 +66,12 @@ module.exports.addTests = function({testRunner, expect, defaultBrowserOptions, p
       const buf = fs.readFileSync(outputFile);
       expect(trace.toString()).toEqual(buf.toString());
     });
+    it('should work without options', async({page, server, outputFile}) => {
+      await page.tracing.start();
+      await page.goto(server.PREFIX + '/grid.html');
+      const trace = await page.tracing.stop();
+      expect(trace).toBeTruthy();
+    });
     it('should return null in case of Buffer error', async({page, server}) => {
       await page.tracing.start({screenshots: true});
       await page.goto(server.PREFIX + '/grid.html');