make private inner classes static

bug found by findbugs
http://b/issue?id=1856695
diff --git a/src/com/android/browser/TabControl.java b/src/com/android/browser/TabControl.java
index e6cacb8..1df4b74 100644
--- a/src/com/android/browser/TabControl.java
+++ b/src/com/android/browser/TabControl.java
@@ -69,7 +69,7 @@
     private final LayoutInflater mInflateService;
     // Subclass of WebViewClient used in subwindows to notify the main
     // WebViewClient of certain WebView activities.
-    private class SubWindowClient extends WebViewClient {
+    private static class SubWindowClient extends WebViewClient {
         // The main WebViewClient.
         private final WebViewClient mClient;