Fix the new generation growth criterion so it is not mislead into performing two growths in a row.

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
diff --git a/runtime/vm/scavenger.h b/runtime/vm/scavenger.h
index 954919ef9be2a5085c994595d4ef35da0528097c..8dffd45c06b80f3711cadbe7eefb95406afd05bd 100644
--- a/runtime/vm/scavenger.h
+++ b/runtime/vm/scavenger.h
@@ -82,9 +82,11 @@ class ScavengeStats {
         promoted_in_words_(promoted_in_words) {}
 
   // Of all data before scavenge, what fraction was found to be garbage?
-  double GarbageFraction() const {
+  // If this scavenge included growth, assume the extra capacity would become
+  // garabge to give the scavenger a chance to stablize at the new capacity.

[danunez, 2017/08/21 18:36:19]

garabge ==> garbage

[rmacnak, 2017/08/22 22:08:45]

Done.

+  double ExpectedGarbageFraction() const {
     intptr_t survived = after_.used_in_words + promoted_in_words_;
-    return 1.0 - (survived / static_cast<double>(before_.used_in_words));
+    return 1.0 - (survived / static_cast<double>(after_.capacity_in_words));
   }
 
   // Fraction of promotion candidates that survived and was thereby promoted.