Viz: Changing OnRegisteredHierarchy

Moving observer calls a bit up in the code so we don't miss hierarchy
events. We could miss them if |parent_source| is nullptr and we return
from the function earlier than events are sent.
Sorry for not catching this earlier.

Bug: 816802
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: Ic72e1b1553df6883540c97ff2cead2240fef7c13
Reviewed-on: https://chromium-review.googlesource.com/1008404Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Illia Martyniuk <illiam@google.com>
Cr-Commit-Position: refs/heads/master@{#550048}

components/viz/service/frame_sinks/frame_sink_manager_impl.cc

@@ -197,6 +197,11 @@ void FrameSinkManagerImpl::RegisterFrameSinkHierarchy(
   DCHECK(!base::ContainsKey(children, child_frame_sink_id));
   children.insert(child_frame_sink_id);
 
+  for (auto& observer : observer_list_) {
+    observer.OnRegisteredFrameSinkHierarchy(parent_frame_sink_id,
+                                            child_frame_sink_id);
+  }
+
   // If the parent has no source, then attaching it to this child will
   // not change any downstream sources.
   BeginFrameSource* parent_source =
@@ -206,11 +211,6 @@ void FrameSinkManagerImpl::RegisterFrameSinkHierarchy(
 
   DCHECK_EQ(registered_sources_.count(parent_source), 1u);
   RecursivelyAttachBeginFrameSource(child_frame_sink_id, parent_source);
-
-  for (auto& observer : observer_list_) {
-    observer.OnRegisteredFrameSinkHierarchy(parent_frame_sink_id,
-                                            child_frame_sink_id);
-  }
 }
 
 void FrameSinkManagerImpl::UnregisterFrameSinkHierarchy(