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

chrome/browser/extensions/extension_apitest.cc

Index: chrome/browser/extensions/extension_apitest.cc
diff --git a/chrome/browser/extensions/extension_apitest.cc b/chrome/browser/extensions/extension_apitest.cc
index 9ab6a72109ea7407ea248444f4cb01af75ef1894..c09e185c340147b98ba114593fb895a7d4d9ca96 100644
--- a/chrome/browser/extensions/extension_apitest.cc
+++ b/chrome/browser/extensions/extension_apitest.cc
@@ -174,6 +174,13 @@ bool ExtensionApiTest::RunPageTest(const std::string& page_url,
 }
 
 bool ExtensionApiTest::RunPlatformAppTest(const char* extension_name) {
+  bool res = RunPlatformAppTestReturnImmediately(extension_name);
+  CloseShellWindowsAndWaitForAppToExit();
+  return res;
+}
+
+bool ExtensionApiTest::RunPlatformAppTestReturnImmediately(
+    const char* extension_name) {
   return RunExtensionTestImpl(extension_name, "", kFlagLaunchPlatformApp);
 }