feat: introduce page.browserContext() (#3655)

lib/Page.js

@@ -172,6 +172,13 @@ class Page extends EventEmitter {
     return this._target.browser();
   }
 
+  /**
+   * @return {!Puppeteer.BrowserContext}
+   */
+  browserContext() {
+    return this._target.browserContext();
+  }
+
   _onTargetCrashed() {
     this.emit('error', new Error('Page crashed!'));
   }