Allow per-platform customization of the default Skia font manager

ci/licenses_golden/licenses_flutter

@@ -711,6 +711,7 @@ FILE: ../../../flutter/synchronization/semaphore_unittest.cc
 FILE: ../../../flutter/third_party/txt/src/txt/platform.cc
 FILE: ../../../flutter/third_party/txt/src/txt/platform.h
 FILE: ../../../flutter/third_party/txt/src/txt/platform_android.cc
+FILE: ../../../flutter/third_party/txt/src/txt/platform_linux.cc
 FILE: ../../../flutter/third_party/txt/src/txt/platform_mac.mm
 FILE: ../../../flutter/vulkan/skia_vulkan_header.h
 FILE: ../../../flutter/vulkan/vulkan_application.cc

lib/ui/text/font_collection.cc

@@ -46,7 +46,7 @@ void _LoadFontFromList(Dart_NativeArguments args) {
 
 FontCollection::FontCollection()
     : collection_(std::make_shared<txt::FontCollection>()) {
-  collection_->SetDefaultFontManager(SkFontMgr::RefDefault());
+  collection_->SetupDefaultFontManager();
 
   dynamic_font_manager_ = sk_make_sp<txt::DynamicFontManager>();
   collection_->SetDynamicFontManager(dynamic_font_manager_);

third_party/txt/BUILD.gn

@@ -123,6 +123,8 @@ source_set("txt") {
     deps += [ "$flutter_root/fml" ]
   } else if (is_android) {
     sources += [ "src/txt/platform_android.cc" ]
+  } else if (is_linux) {
+    sources += [ "src/txt/platform_linux.cc" ]
   } else {
     sources += [ "src/txt/platform.cc" ]
   }

third_party/txt/src/txt/font_collection.cc

@@ -87,8 +87,8 @@ size_t FontCollection::GetFontManagersCount() const {
   return GetFontManagerOrder().size();
 }
 
-void FontCollection::SetDefaultFontManager(sk_sp<SkFontMgr> font_manager) {
-  default_font_manager_ = font_manager;
+void FontCollection::SetupDefaultFontManager() {
+  default_font_manager_ = GetDefaultFontManager();
 }
 
 void FontCollection::SetAssetFontManager(sk_sp<SkFontMgr> font_manager) {

third_party/txt/src/txt/font_collection.h

@@ -40,7 +40,7 @@ class FontCollection : public std::enable_shared_from_this<FontCollection> {
 
   size_t GetFontManagersCount() const;
 
-  void SetDefaultFontManager(sk_sp<SkFontMgr> font_manager);
+  void SetupDefaultFontManager();
   void SetAssetFontManager(sk_sp<SkFontMgr> font_manager);
   void SetDynamicFontManager(sk_sp<SkFontMgr> font_manager);
   void SetTestFontManager(sk_sp<SkFontMgr> font_manager);

third_party/txt/src/txt/platform.cc

@@ -10,4 +10,8 @@ std::string GetDefaultFontFamily() {
   return "Arial";
 }
 
+sk_sp<SkFontMgr> GetDefaultFontManager() {
+  return SkFontMgr::RefDefault();
+}
+
 }  // namespace txt

third_party/txt/src/txt/platform.h

@@ -8,10 +8,14 @@
 #include <string>
 #include "flutter/fml/macros.h"
 
+#include "third_party/skia/include/core/SkFontMgr.h"
+
 namespace txt {
 
 std::string GetDefaultFontFamily();
 
+sk_sp<SkFontMgr> GetDefaultFontManager();
+
 }  // namespace txt
 
 #endif  // TXT_PLATFORM_H_

third_party/txt/src/txt/platform_android.cc

@@ -10,4 +10,8 @@ std::string GetDefaultFontFamily() {
   return "sans-serif";
 }
 
+sk_sp<SkFontMgr> GetDefaultFontManager() {
+  return SkFontMgr::RefDefault();
+}
+
 }  // namespace txt

third_party/txt/src/txt/platform_linux.cc

@@ -0,0 +1,19 @@
+// Copyright 2013 The Flutter 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 "txt/platform.h"
+
+#include "third_party/skia/include/ports/SkFontMgr_directory.h"
+
+namespace txt {
+
+std::string GetDefaultFontFamily() {
+  return "Arial";
+}
+
+sk_sp<SkFontMgr> GetDefaultFontManager() {
+  return SkFontMgr_New_Custom_Directory("/usr/share/fonts/");
+}
+
+}  // namespace txt

third_party/txt/src/txt/platform_mac.mm

@@ -24,4 +24,8 @@
   }
 }
 
+sk_sp<SkFontMgr> GetDefaultFontManager() {
+  return SkFontMgr::RefDefault();
+}
+
 }  // namespace txt