No longer start BG mode until a BackgroundContents is loaded

chrome/test/data/extensions/api_test/app_background_page/common/close.html

+<script src="common.js"></script>
+<h1 id="status">Close BG Page</h1>
+<div style="display:none" id="pageToScriptTunnel"></div>
+<div style="display:none" id="scriptToPageTunnel"></div>
+
+<script>
+var backgroundWindow;
+
+window.onload = function() {
+  // This page just closes an existing BG window.
+  setupScriptTunnel();
+  backgroundWindow = window.open('bg.html', 'bg', 'background');
+  if (backgroundWindow) {
+    setStatus('closing background page');
+    backgroundWindow.close();
+    notifyBackgroundPageClosed();
+  } else {
+    notifyBackgroundPagePermissionDenied();
+    setStatus('background page permission denied');
+  }
+}
+</script>