Add 6dof to the list of supported gvr features

While reimplementing gvr_experimental_is_feature_supported() as
gvr_is_feature_supported() I accidentally removed 6dof from the list of
supported features.

Bug: 34742071
Test: -
Change-Id: Ic0c9754d72a704d6bfd74dd7a14bdf6e89ed4dd4
1 file changed