Replace domWindow with GetExecutionContext in TextTrackLoader.

Document::domWindow() can be null when a Document is constructed without
a browsing context. To prevent this possible derference, use
Document::GetExecutionContext which is guaranteed to be non-null.

BUG=1116743

Change-Id: Ib679e9689692f6b24a046ba43a17e16b2db858f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2360493
Auto-Submit: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799154}

third_party/blink/renderer/core/loader/text_track_loader.cc

@@ -108,7 +108,8 @@ bool TextTrackLoader::Load(const KURL& url,
                            CrossOriginAttributeValue cross_origin) {
   CancelLoad();
 
-  ResourceLoaderOptions options(GetDocument().domWindow()->GetCurrentWorld());
+  ResourceLoaderOptions options(
+      GetDocument().GetExecutionContext()->GetCurrentWorld());
   options.initiator_info.name = fetch_initiator_type_names::kTrack;
 
   // Let |request| be the result of creating a potential-CORS request