[heap] Fix computation of marking progress in incremental finalization.

The existing formula returns non-sense when the old top is greater than
the current top.

BUG=chromium:694255

Review-Url: https://codereview.chromium.org/2850373003
Cr-Commit-Position: refs/heads/master@{#45037}

src/heap/incremental-marking.cc

@@ -711,8 +711,6 @@ void IncrementalMarking::FinalizeIncrementally() {
 
   double start = heap_->MonotonicallyIncreasingTimeInMs();
 
-  int old_marking_deque_top = marking_deque()->top();
-
   // After finishing incremental marking, we try to discover all unmarked
   // objects to reduce the marking load in the final pause.
   // 1) We scan and mark the roots again to find all changes to the root set.
@@ -728,10 +726,10 @@ void IncrementalMarking::FinalizeIncrementally() {
   }
   ProcessWeakCells();
 
-  int marking_progress = abs(old_marking_deque_top - marking_deque()->top());
-
-  marking_progress += static_cast<int>(
-      heap_->local_embedder_heap_tracer()->NumberOfCachedWrappersToTrace());
+  int marking_progress =
+      heap_->mark_compact_collector()->marking_deque()->Size() +
+      static_cast<int>(
+          heap_->local_embedder_heap_tracer()->NumberOfCachedWrappersToTrace());
 
   double end = heap_->MonotonicallyIncreasingTimeInMs();
   double delta = end - start;

src/heap/sequential-marking-deque.h

@@ -47,6 +47,11 @@ class SequentialMarkingDeque {
 
   inline bool IsEmpty() { return top_ == bottom_; }
 
+  int Size() {
+    // Return (top - bottom + capacity) % capacity, where capacity = mask + 1.
+    return (top_ - bottom_ + mask_ + 1) & mask_;
+  }
+
   bool overflowed() const { return overflowed_; }
 
   void ClearOverflowed() { overflowed_ = false; }
@@ -114,8 +119,6 @@ class SequentialMarkingDeque {
     top_ = new_top;
   }
 
-  int top() { return top_; }
-
  private:
   // This task uncommits the marking_deque backing store if
   // markin_deque->in_use_ is false.