Now closes existing background contents if app opens a new one.

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

+<script src="common.js"></script>
+<h1 id="status">Page C</h1>
+<div style="display:none" id="pageToScriptTunnel"></div>
+<div style="display:none" id="scriptToPageTunnel"></div>
+
+<script>
+var backgroundWindow;
+
+window.onload = function() {
+  setupScriptTunnel();
+  // Note we are using a different window name than a.html so this should open
+  // a different background page.
+  backgroundWindow = window.open('bg.html', 'bg2', 'background');
+  if (backgroundWindow) {
+    setStatus('background page opened');
+  } else {
+    notifyBackgroundPagePermissionDenied();
+    setStatus('background page permission denied');
+  }
+}
+
+function onBackgroundPageLoaded() {
+  if (!backgroundWindow.checkAndIncrementCounter(0)) {
+    notifyCounterError();
+    return;
+  }
+
+  setStatus('background page loaded');
+  notifyBackgroundPageLoaded();
+}
+</script>