Use a generic sans-serif font family on Android that will be recogniz…

…ed by Skia's font manager (flutter#4055)

third_party/txt/BUILD.gn

@@ -105,6 +105,8 @@ source_set("txt") {
     set_sources_assignment_filter([])
     sources += [ "src/txt/platform_mac.mm" ]
     set_sources_assignment_filter(sources_assignment_filter)
+  } else if (is_android) {
+    sources += [ "src/txt/platform_android.cc" ]
   } else {
     sources += [ "src/txt/platform.cc" ]
   }

third_party/txt/src/txt/paragraph.cc

@@ -148,8 +148,8 @@ bool Paragraph::AddRunsToLineBreaker(
     auto collection = font_collection_->GetMinikinFontCollectionForFamily(
         run.style.font_family);
     if (collection == nullptr) {
-      FTL_DLOG(INFO) << "Could not find font collection for family \""
-                     << run.style.font_family << "\".";
+      FTL_LOG(INFO) << "Could not find font collection for family \""
+                    << run.style.font_family << "\".";
       return false;
     }
     breaker_.addStyleRun(&paint, collection, font, run.start, run.end, false,

third_party/txt/src/txt/platform_android.cc

@@ -0,0 +1,13 @@
+// Copyright 2017 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 "txt/platform.h"
+
+namespace txt {
+
+std::string GetDefaultFontFamily() {
+  return "sans-serif";
+}
+
+}  // namespace txt

travis/licenses_golden/licenses_flutter

@@ -1194,6 +1194,7 @@ FILE: ../../../flutter/third_party/txt/src/txt/directory_asset_data_provider.cc
 FILE: ../../../flutter/third_party/txt/src/txt/directory_asset_data_provider.h
 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_mac.mm
 FILE: ../../../flutter/vulkan/vulkan_native_surface_magma.cc
 FILE: ../../../flutter/vulkan/vulkan_native_surface_magma.h