[infra] Translate _sources.gypi files to _sources.gni files

runtime/lib/math_sources.gni

Index: runtime/lib/math_sources.gni
diff --git a/runtime/lib/math_sources.gypi b/runtime/lib/math_sources.gni
similarity index 54%
rename from runtime/lib/math_sources.gypi
rename to runtime/lib/math_sources.gni
index 2a181738ad5bd6f8f8b2a4dd84259bcd843dddde..65a07ca5a8e589e878bd41d4e0c25166cbb19cc9 100644
--- a/runtime/lib/math_sources.gypi
+++ b/runtime/lib/math_sources.gni
@@ -1,10 +1,8 @@
-# Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+# Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
-{
-  'sources': [
-    'math.cc',
-    'math_patch.dart',
-  ],
-}
+math_runtime_sources = [
+  "math.cc",
+  "math_patch.dart",
+]