[snapshot] add comment for TargetChunkSize

R=jgruber@chromium.org

Change-Id: If9b235868905b818912109f0684b888335c75e80
Reviewed-on: https://chromium-review.googlesource.com/1125845Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54204}

src/snapshot/default-serializer-allocator.h

@@ -35,6 +35,8 @@ class DefaultSerializerAllocator final {
   void OutputStatistics();
 
  private:
+  // We try to not exceed this size for every chunk. We will not succeed for
+  // larger objects though.
   uint32_t TargetChunkSize(int space);
 
   static constexpr int kNumberOfPreallocatedSpaces =