feat(Tracing): allow custom tracing categories (#1439)

This patch adds `categories` option to the `page.tracing.start` method
that allows to override default tracing categories. 

References #1300, #854

docs/api.md

@@ -1334,6 +1334,7 @@ await page.tracing.stop();
 - `options` <[Object]>
   - `path` <[string]> A path to write the trace file to. **required**
   - `screenshots` <[boolean]> captures screenshots in the trace.
+  - `categories` <[Array]<[string]>> specify custom categories to use instead of default. 
 - returns: <[Promise]>
 
 Only one trace can be active at a time per browser.

lib/Tracing.js

@@ -37,12 +37,13 @@ class Tracing {
     console.assert(!this._recording, 'Cannot start recording trace while already recording trace.');
     console.assert(options.path, 'Must specify a path to write trace file to.');
 
-    const categoriesArray = [
+    const defaultCategories = [
       '-*', 'devtools.timeline', 'v8.execute', 'disabled-by-default-devtools.timeline',
       'disabled-by-default-devtools.timeline.frame', 'toplevel',
       'blink.console', 'blink.user_timing', 'latencyInfo', 'disabled-by-default-devtools.timeline.stack',
       'disabled-by-default-v8.cpu_profiler'
     ];
+    const categoriesArray = options.categories || defaultCategories;
 
     if (options.screenshots)
       categoriesArray.push('disabled-by-default-devtools.screenshot');

test/test.js

@@ -3005,6 +3005,13 @@ describe('Page', function() {
       await page.tracing.stop();
       expect(fs.existsSync(outputFile)).toBe(true);
     }));
+    it('should run with custom categories if provided', SX(async function() {
+      await page.tracing.start({path: outputFile, categories: ['disabled-by-default-v8.cpu_profiler.hires']});
+      await page.tracing.stop();
+
+      const traceJson = JSON.parse(fs.readFileSync(outputFile));
+      expect(traceJson.metadata['trace-config']).toContain('disabled-by-default-v8.cpu_profiler.hires');
+    }));
     it('should throw if tracing on two pages', SX(async function() {
       await page.tracing.start({path: outputFile});
       const newPage = await browser.newPage();