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>