Add Page.browser() method (#2387)

Call page.browser() to get the browser instance associated with a
page.

Fixes #2275

test/page.spec.js

@@ -1562,4 +1562,10 @@ module.exports.addTests = function({testRunner, expect, puppeteer, DeviceDescrip
       await closedPromise;
     });
   });
+
+  describe('Page.browser', function() {
+    it('should return the correct browser instance', async function({ page, browser }) {
+      expect(page.browser()).toBe(browser);
+    });
+  });
 };