Checks FontSelector is being used in the proper place

Bug: 988125
Change-Id: I015694f778f7f8ae2a58a667f45b2aff184abf19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776221
Auto-Submit: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691650}

third_party/blink/renderer/core/css/font_face_set_document.h

@@ -75,6 +75,8 @@ class CORE_EXPORT FontFaceSetDocument final : public FontFaceSet,
  protected:
   bool InActiveContext() const override;
   FontSelector* GetFontSelector() const override {
+    // TODO(Fserb): tracking down crbug.com/988125, can be DCHECK later.
+    CHECK(IsMainThread());
     return GetDocument()->GetStyleEngine().GetFontSelector();
   }
 

third_party/blink/renderer/core/css/font_face_set_worker.h

@@ -12,6 +12,7 @@
 #include "third_party/blink/renderer/core/css/font_face_set.h"
 #include "third_party/blink/renderer/core/css/offscreen_font_selector.h"
 #include "third_party/blink/renderer/core/workers/worker_global_scope.h"
+#include "third_party/blink/renderer/core/workers/worker_thread.h"
 #include "third_party/blink/renderer/platform/heap/handle.h"
 #include "third_party/blink/renderer/platform/wtf/allocator/allocator.h"
 
@@ -49,6 +50,8 @@ class CORE_EXPORT FontFaceSetWorker final
  protected:
   bool InActiveContext() const override { return true; }
   FontSelector* GetFontSelector() const override {
+    // TODO(Fserb): tracking down crbug.com/988125, can be DCHECK later.
+    CHECK(GetWorker()->GetThread()->IsCurrentThread());
     return GetWorker()->GetFontSelector();
   }
   // For workers, this is always an empty list.