Check tree view is ready when creating DedicatedWorker

This is needed to get frame sink ids for DedicatedWorkers' RAF.

Bug: 863698
Change-Id: I0bd86ac9271d3f00ee5cb00522896ff152ef7bb6
Reviewed-on: https://chromium-review.googlesource.com/1140278Reviewed-by: Justin Novosad <junov@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575762}

third_party/blink/renderer/core/workers/dedicated_worker.cc

@@ -185,14 +185,16 @@ BeginFrameProviderParams DedicatedWorker::CreateBeginFrameProviderParams() {
   // won't be initialized. If that's the case, the Worker will initialize it by
   // itself later.
   BeginFrameProviderParams begin_frame_provider_params;
-  if (GetExecutionContext()->IsDocument()) {
+  if (GetExecutionContext() && GetExecutionContext()->IsDocument()) {
     LocalFrame* frame = ToDocument(GetExecutionContext())->GetFrame();
     WebLayerTreeView* layer_tree_view = nullptr;
-    if (frame) {
+    if (frame && frame->GetPage()) {
       layer_tree_view =
           frame->GetPage()->GetChromeClient().GetWebLayerTreeView(frame);
-      begin_frame_provider_params.parent_frame_sink_id =
-          layer_tree_view->GetFrameSinkId();
+      if (layer_tree_view) {
+        begin_frame_provider_params.parent_frame_sink_id =
+            layer_tree_view->GetFrameSinkId();
+      }
     }
     begin_frame_provider_params.frame_sink_id =
         Platform::Current()->GenerateFrameSinkId();