Implement WebSettings.{get|set}TextSize via {get|set}TextZoom.

TextSize is deprecated, so we wouldn't expect from WebViewProvider
implementations to re-implement it in some other way than Android WebView does
it. This also makes recently added TextSize.getValue method redundant.

Change-Id: Ia9d0c743424be0727be6d5df6c7da30514d621b7