Fix an assert failure in art::DumpNativeStack().

When ThreadList::SuspendAll() times out (and aborts),
UnsafeLogFatalForThreadSuspendAllTimeout() will call
art::DumpNativeStack() but it does not have the mutator lock
shared-locked (as it failed while trying to exclusive-lock the mutator
lock) and the AssertSharedHeld() on the mutator lock fails. It's an
assert failure nested in an time-out abort.

This change avoids it by letting it print the native method frame info
only if the thread has a shared lock on the mutator lock, as opposed
to asserting that it has a shared lock.

Change-Id: I17851ebcaa37f4f67086c15243a2cffea3997a02