[heap] ObjectStats: Fix off-by-one in histogram

BUG=chromium:631094
R=hpayer@chromium.org

Review-Url: https://codereview.chromium.org/2181373002
Cr-Commit-Position: refs/heads/master@{#38067}

src/heap/object-stats.h

@@ -94,7 +94,7 @@ class ObjectStats {
   static const int kLastBucketShift = 19;  // >512k
   static const int kFirstBucket = 1 << kFirstBucketShift;
   static const int kLastBucket = 1 << kLastBucketShift;
-  static const int kNumberOfBuckets = kLastBucketShift - kFirstBucketShift;
+  static const int kNumberOfBuckets = kLastBucketShift - kFirstBucketShift + 1;
 
   int HistogramIndexFromSize(size_t size) {
     if (size == 0) return 0;