Allow multiple calls to SetDefaultSkiaFactory on Windows.

skia/ext/fontmgr_default_android.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "skia/ext/fontmgr_default_android.h"
 
 #include "third_party/skia/include/ports/SkFontMgr.h"
 #include "third_party/skia/include/ports/SkFontMgr_android.h"
 
 namespace {
 // An owning leaky bare pointer.
 SkFontMgr* g_default_fontmgr;
 }  // namespace
 
 SK_API void SetDefaultSkiaFactory(sk_sp<SkFontMgr> fontmgr) {
+  SkASSERT(g_default_fontmgr == nullptr);
   g_default_fontmgr = fontmgr.release();
 }
 
 SK_API sk_sp<SkFontMgr> SkFontMgr::Factory() {
   return g_default_fontmgr ? sk_ref_sp(g_default_fontmgr)
                            : SkFontMgr_New_Android(nullptr);
 }