Add an empty typeface for testing

Change-Id: I091389fcc41b9eb5002ae0d005360249b51884d9
Reviewed-on: https://skia-review.googlesource.com/150137
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 3369a9a..4350eb6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1351,6 +1351,7 @@
       "tools/debugger/SkDrawCommand.cpp",
       "tools/debugger/SkJsonWriteBuffer.cpp",
       "tools/fonts/SkRandomScalerContext.cpp",
+      "tools/fonts/SkTestEmptyTypeface.h",
       "tools/fonts/SkTestFontMgr.cpp",
       "tools/fonts/SkTestFontMgr.h",
       "tools/fonts/SkTestSVGTypeface.cpp",