Bug 1675853 - FontFace.load() doesn't need to flush the user fontset. r=heycam

This one is even more subtle. This could cause a call from SetStatus()
to OnFontFaceStatusChange() to not be done as part of the load()
function (if the face is about to get added to the set or such).

However, when it's actually added via UpdateRules, we'd properly
invalidate the relevant state as well, so I think this is also fine, and
our existing tests also agree.

Depends on D96342

Differential Revision: https://phabricator.services.mozilla.com/D96343

layout/style/FontFace.cpp

@@ -328,8 +328,6 @@ FontFaceLoadStatus FontFace::Status() { return mStatus; }
 Promise* FontFace::Load(ErrorResult& aRv) {
   MOZ_ASSERT(NS_IsMainThread());
 
-  mFontFaceSet->FlushUserFontSet();
-
   EnsurePromise();
 
   if (!mLoaded) {