Persist the scanning state across invocations of ProcessToSpace.

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
diff --git a/runtime/vm/scavenger.h b/runtime/vm/scavenger.h
index 9eb3c4ba2f12c876cab0bcce19bf61705b143505..62e7a129e6d29672128a44bf809f2ca011190856 100644
--- a/runtime/vm/scavenger.h
+++ b/runtime/vm/scavenger.h
@@ -117,6 +117,10 @@ class Scavenger {
   uword top_;
   uword end_;
 
+  // A pointer to the first unscanned object.  Scanning completes when
+  // this value meets the allocation top.
+  uword resolved_top_;
+
   // Objects below this address have survived a scavenge.
   uword survivor_end_;