devtools: Fix distances when no user roots have been found

In the case that user roots are not provided, distances should start
at 0. This is the case when full heap snapshots are provided with
the gn flag
  v8_enable_full_heap_snapshots

For production builds this flag is always false.

V8 CL: https://crrev.com/c/1528994

Bug: 936797
Change-Id: Ifd247a082a4336b59d05f444ba54d281f9eae805
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529096Reviewed-by: Alexei Filippov <alph@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642413}

third_party/blink/renderer/devtools/front_end/heap_snapshot_worker/HeapSnapshot.js

@@ -1305,7 +1305,7 @@ HeapSnapshotWorker.HeapSnapshot = class {
     this._bfs(nodesToVisit, nodesToVisitLength, distances, filter);
 
     // BFS for objects not reached from user roots.
-    distances[this.rootNode().ordinal()] = HeapSnapshotModel.baseSystemDistance;
+    distances[this.rootNode().ordinal()] = nodesToVisitLength > 0 ? HeapSnapshotModel.baseSystemDistance : 0;
     nodesToVisit[0] = this.rootNode().nodeIndex;
     nodesToVisitLength = 1;
     this._bfs(nodesToVisit, nodesToVisitLength, distances, filter);