Make CHECK -> DCHECK as per comment

Bug: 988125
Change-Id: Iaec7cd80f39849da0a476c1653655b68cef1452e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2529103
Auto-Submit: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827786}

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

@@ -81,8 +81,7 @@ 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());
+    DCHECK(IsMainThread());
     return GetDocument()->GetStyleEngine().GetFontSelector();
   }