Early-exit from RemoteFontFaceSource::BeginLoadIfNeeded if the ExecutionContext is detached

Bug: 1084831
Change-Id: Ia145d711cc71fdfc698e560ad67e1723390025c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210005Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770849}

third_party/blink/renderer/core/css/remote_font_face_source.cc

@@ -359,7 +359,7 @@ RemoteFontFaceSource::CreateLoadingFallbackFontData(
 }
 
 void RemoteFontFaceSource::BeginLoadIfNeeded() {
-  if (IsLoaded())
+  if (IsLoaded() || !font_selector_->GetExecutionContext())
     return;
   DCHECK(GetResource());