Bug 1028497 - Part 16: Don't destroy the user font set if there are no @font-face rules. r=jfkthame

We can no longer call FontFaceSet::DestroyUserFontSet (what used to be
nsUserFontSet::Destroy) in nsPresContext::FlushUserFontSet, since we
need to keep tracking FontFace objects even if the list of @font-face
rules is empty.

layout/base/nsPresContext.cpp

@@ -2109,26 +2109,14 @@ nsPresContext::FlushUserFontSet()
     if (gfxPlatform::GetPlatform()->DownloadableFontsEnabled()) {
       nsTArray<nsFontFaceRuleContainer> rules;
       if (!mShell->StyleSet()->AppendFontFaceRules(this, rules)) {
-        if (mFontFaceSet) {
-          mFontFaceSet->DestroyUserFontSet();
-        }
         return;
       }
 
-      bool changed = false;
-
-      if (rules.Length() == 0) {
-        if (mFontFaceSet) {
-          mFontFaceSet->DestroyUserFontSet();
-          changed = true;
-        }
-      } else {
-        if (!mFontFaceSet) {
-          mFontFaceSet = new FontFaceSet(mDocument->GetInnerWindow(), this);
-        }
-        mFontFaceSet->EnsureUserFontSet(this);
-        changed = mFontFaceSet->UpdateRules(rules);
+      if (!mFontFaceSet) {
+        mFontFaceSet = new FontFaceSet(mDocument->GetInnerWindow(), this);
       }
+      mFontFaceSet->EnsureUserFontSet(this);
+      bool changed = mFontFaceSet->UpdateRules(rules);
 
       // We need to enqueue a style change reflow (for later) to
       // reflect that we're modifying @font-face rules.  (However,