Useragent switcher tweaks

 Children inherit parent tab's setting
 Setting is now save/restored across kills

Change-Id: I664d2d887281df7b99e8239e3ba41ba540f4bbec
diff --git a/src/com/android/browser/BrowserSettings.java b/src/com/android/browser/BrowserSettings.java
index dd46314..46d47b0 100644
--- a/src/com/android/browser/BrowserSettings.java
+++ b/src/com/android/browser/BrowserSettings.java
@@ -399,6 +399,9 @@
     }
 
     public void toggleDesktopUseragent(WebView view) {
+        if (view == null) {
+            return;
+        }
         WebSettings settings = view.getSettings();
         if (mCustomUserAgents.get(settings) != null) {
             mCustomUserAgents.remove(settings);