Visit deoptimized shadow frames as roots

During deoptimization, we create shadow frames but do not attach them to the
stack until we transition to interpreter mode. If a GC happens before that,
these shadow frames are not visited by GC so they may contain stale references.

This CL addresses this issue by visiting the shadow frames "under contruction"
in Thread::VisitRoots so we correctly update all references they hold.

To make them visible, we now save the top shadow frame (the first one created)
in the field Thread::tls_ptr_sized_values::deoptimization_shadow_frame. This
field will then be cleared when transitioning to interpreter mode.

Bug: 14324885
Change-Id: Ib213ddc80f19cfcdfcec6f20acaa7f1a0e9ce2c1