Bug 1475139 part 5 - Only use external fonts with DrawEventRecorderMemory if we have a callback. r=jrmuizel

This was giving me some font assertion crashes, and changing this as a hunch fixed it.

Differential Revision: https://phabricator.services.mozilla.com/D6784

--HG--
extra : rebase_source : 99bf039f33e314fb6f88ea283cf4cc575b054566

gfx/2d/DrawEventRecorder.cpp

@@ -114,7 +114,7 @@ DrawEventRecorderMemory::DrawEventRecorderMemory()
 DrawEventRecorderMemory::DrawEventRecorderMemory(const SerializeResourcesFn &aFn) :
   mSerializeCallback(aFn)
 {
-  mExternalFonts = true;
+  mExternalFonts = !!mSerializeCallback;
   WriteHeader(mOutputStream);
 }