Make WebView.HitTestResult static

- we can do this, because HitTestResult never had a published constructor
  so the only code that can create it has always lived in this package.
- doing this makes the class design cleaner, and smooths the way for allowing
  the new WebView implementation to deal in these results in contexts where
  the appropriate WebView instance is not readily available.

Change-Id: I9e1f1c0faeb16436b1861aab3c2871a5851dfb54