Fix possible random NPE in AbstractWebViewBridgeFragment

* AbstractWebViewBridgeFragment.onDestroyView() calls WebView.destroy().
* AbstractWebViewBridgeFragment.onSaveInstanceState() calls
  WebView.saveState().
* WebView in at least Android 4.0 seems to throw NPE when saveState()
  called after destroy().
* onSaveInstanceState() is NOT guaranteed to be called before
  onDestroyView().
* Hense, random crash may occur.

Refer:
http://stackoverflow.com/questions/15313598/once-for-all-how-to-correctly-save-instance-state-of-fragments-in-back-stack
Fix not clearing webview afeter destroy
Remove unclarified webview.saveState logic
Merge pull request #29 from ypresto/fix_crash_webview_fragment

Fix possible random NPE in AbstractWebViewBridgeFragment