Fix DCHECK failures from Class::VisitFieldRoots

We now use GetDeclaringClassUnchecked when marking roots to fix
flaky test failures. Fixed a race condition in root marking where
we could have non zero field array length with a null pointer.
Fixed a race condition where we could be marking roots before
FixupTemporaryDeclaringClass had finished. The solution is to
only do the declaring class CHECK if we are at least resolved.
Fixed JDWP tests by changing FieldId / MethodId to be 64 bits.

Also some cleanup.

Change-Id: Ibac09519860d93c3f68a5cc964bbc91dc10a279a