Change Checkpoint API to return total number of checkpoints

Fixes a race condition with SetStateUnsafe that caused some
warnings in the Barrier::~Barrier.

The race was:
RunCheckpoint sees suspended thread, runs the checkpoint. Inside the
checkpoint, the thread state had changed to runnable by
SetStateUnsafe. This occasionally caused more Barrier::Pass than
expected.

The fix is to return the total number of checkpoints instead of just
the runnable ones.

Bug: 24191051
Change-Id: If15a933ed4c8efa66a5f27cd5feaa2e5957ae804
diff --git a/runtime/gc/heap.cc b/runtime/gc/heap.cc
index 1d38525..ab93142 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -1291,9 +1291,7 @@
     ATRACE_END();
     // If thread is a running mutator, then act on behalf of the trim thread.
     // See the code in ThreadList::RunCheckpoint.
-    if (thread->GetState() == kRunnable) {
-      barrier_->Pass(Thread::Current());
-    }
+    barrier_->Pass(Thread::Current());
   }
 
  private: