Add a read barrier for weak roots in monitors.

A weak root requires a read barrier for the to-space invariant to hold
because the object pointed to by a weak root can't be marked/forwarded
like the one pointed to by a strong root (GC does not know if it's
alive or not at that point) and because, without a read barrier, a
mutator could access it and obtain a from-space reference, which would
violate the to-space invariant.

TODO: do similar for the other types of weak roots.

Bug: 12687968
Change-Id: I563a0fa4f875e0c21ac96f57696959454e13b15a