Add 'getValue' methods to WebSettings.TextSize and ZoomDensity enums.

As the 'value' fields in these enums are not public, it isn't possible to
retrieve the enum value if the WebViewProvider implementation is defined in a
different package to the API. Adding public hidden methods to fix this.

Change-Id: Ia5fb1332d0043e127b4bad03a295d8abf1a5e5b9