Only allow to intercept navigation for main frame and subframe

The interception for resource request should be restricted to main frame
and sub frames, so that the page loading state change events, e.g.
onPageStarted and onPageFinished, are triggered only for main frame and sub
frame instead of each kind of resources.

BUG=https://crosswalk-project.org/jira/browse/XWALK-306
Merge pull request #2 from hmin/navigation-interception-xdk

Only allow to intercept navigation for main frame and subframe