feat(Page): add option to run 'beforeunload' when closing the page (#2478)

Today, `page.close()` method doesn't run page's beforeunload listeners.
This way users can be sure that `page.close()` actually closes the
page.

This patch adds an optional `runBeforeUnload` option to the
`page.close()` method that would run beforeunload listeners. Note:
running beforeunload handlers might cancel page closing.

Fixes #2386.

test/assets/beforeunload.html

@@ -0,0 +1,5 @@
+<script>
+window.addEventListener('beforeunload', event => {
+  event.returnValue = 'Leave?';
+});
+</script>

test/page.spec.js

@@ -40,6 +40,20 @@ module.exports.addTests = function({testRunner, expect, puppeteer, DeviceDescrip
       await newPage.close();
       expect(await browser.pages()).not.toContain(newPage);
     });
+    it('should run beforeunload if asked for', async({browser, server}) => {
+      const newPage = await browser.newPage();
+      await newPage.goto(server.PREFIX + '/beforeunload.html');
+      // We have to interact with a page so that 'beforeunload' handlers
+      // fire.
+      await newPage.click('body');
+      newPage.close({ runBeforeUnload: true });
+      const dialog = await waitEvent(newPage, 'dialog');
+      expect(dialog.type()).toBe('beforeunload');
+      expect(dialog.defaultValue()).toBe('');
+      expect(dialog.message()).toBe('');
+      dialog.accept();
+      await waitEvent(newPage, 'close');
+    });
   });
 
   describe('Page.Events.error', function() {