Keep browser process alive while there are platform apps with background pages running.

chrome/test/data/extensions/platform_apps/open_link/main.js

Index: chrome/test/data/extensions/platform_apps/open_link/main.js
diff --git a/chrome/test/data/extensions/platform_apps/open_link/main.js b/chrome/test/data/extensions/platform_apps/open_link/main.js
index ddd0c8a6a100a61185b4e2ae1bdd876efb2d7ec1..adb7f0e4ebd980bfe74ec56246aaf9fd7728a694 100644
--- a/chrome/test/data/extensions/platform_apps/open_link/main.js
+++ b/chrome/test/data/extensions/platform_apps/open_link/main.js
@@ -17,3 +17,5 @@ chrome.test.getConfig(function(config) {
 onmessage = function() {
   chrome.test.sendMessage('Link opened');
 };
+
+chrome.test.sendMessage('Launched');