libweave: TypedValueToJson returns derived class not base::Value

No reason to return base class and lose type information.

BUG=brillo:1245
TEST='FEATURES=test emerge-gizmo buffet'

Change-Id: Ia554e902db9ca2ebaaaaf224b3c402f4a915c55c
Reviewed-on: https://chromium-review.googlesource.com/287593
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>