Add diagnostic CHECKs for deserialization error.

A crash has been traced back to a deserialization error on the
viz.mojom.FrameSinkManager RegisterFrameSinkHierarchy IPC. Add some
CHECKs to identify the root cause of bad FrameSinkId(s) being sent.

Bug: 1115094
Change-Id: Ie8aaf54bdddfb6d649b1386ac62edfb0df639cff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2403581Reviewed-by: Jonathan Backer <backer@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805836}

components/viz/host/host_frame_sink_manager.cc

@@ -195,6 +195,11 @@ bool HostFrameSinkManager::RegisterFrameSinkHierarchy(
     return false;
   }
 
+  // TODO(crbug.com/1115094): Convert to DCHECKs after root cause of the
+  // deserialization error is identified.
+  CHECK(parent_frame_sink_id.is_valid()) << parent_frame_sink_id;
+  CHECK(child_frame_sink_id.is_valid()) << child_frame_sink_id;
+
   // Register and store the parent.
   frame_sink_manager_->RegisterFrameSinkHierarchy(parent_frame_sink_id,
                                                   child_frame_sink_id);