Reduce GC log spam.

Homogeneous compaction and collector transitions now print to
VLOG(gc) instead of LOG(INFO).

Bug: 17166556
Change-Id: I5d54acf36a9b2763922e6329596a2f191d45535c
diff --git a/runtime/gc/heap.cc b/runtime/gc/heap.cc
index 47e1709..8f568b1 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -1576,7 +1576,7 @@
   count_performed_homogeneous_space_compaction_++;
   // Print statics log and resume all threads.
   uint64_t duration = NanoTime() - start_time;
-  LOG(INFO) << "Heap homogeneous space compaction took " << PrettyDuration(duration) << " size: "
+  VLOG(gc) << "Heap homogeneous space compaction took " << PrettyDuration(duration) << " size: "
             << PrettySize(space_size_before_compaction) << " -> "
             << PrettySize(space_size_after_compaction) << " compact-ratio: "
             << std::fixed << static_cast<double>(space_size_after_compaction) /
@@ -1727,7 +1727,7 @@
   } else {
     saved_str = " expanded " + PrettySize(-delta_allocated);
   }
-  LOG(INFO) << "Heap transition to " << process_state_ << " took "
+  VLOG(gc) << "Heap transition to " << process_state_ << " took "
       << PrettyDuration(duration) << saved_str;
 }