DevTools: no link when snapshot loaded from file

Code failed when snapshot was not taken from the running application but
loaded from file. heapProfilerModel() returns null if snapshot was loaded
from file.

Bug: chromium:854097
Change-Id: I3c269a78f7f8ec59a15f510dda6ab55a72f53616
Reviewed-on: https://chromium-review.googlesource.com/1146921
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577515}

third_party/blink/renderer/devtools/front_end/profiler/HeapSnapshotView.js

@@ -209,6 +209,9 @@ Profiler.HeapSnapshotView = class extends UI.SimpleView {
    */
   async linkifyObject(snapshotObjectId) {
     const heapProfilerModel = this._profile.heapProfilerModel();
+    // heapProfilerModel is null if snapshot was loaded from file
+    if (!heapProfilerModel)
+      return null;
     const remoteObject = await heapProfilerModel.objectForSnapshotObjectId(String(snapshotObjectId), 'link');
     if (!remoteObject || remoteObject.type !== 'function')
       return null;