call portable font cleanup only once at app end

R=mtklein@google.com

Review URL: https://codereview.chromium.org/1180593005
diff --git a/dm/DM.cpp b/dm/DM.cpp
index af20a78..33ce0a4 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -797,6 +797,7 @@
     }
     tg.wait();
     // At this point we're back in single-threaded land.
+    sk_tool_utils::release_portable_typefaces();
 
     SkDebugf("\n");
     if (gFailures.count() > 0) {