Set the ttcIndex on Mac onOpenStream.

Since on Mac onOpenStream always creates a new stream of a
non-collection font, be sure the out ttcIndex is always set to 0.

Review URL: https://codereview.chromium.org/975473002

src/ports/SkFontHost_mac.cpp

@@ -1740,6 +1740,7 @@ SkStreamAsset* SkTypeface_Mac::onOpenStream(int* ttcIndex) const {
         ++entry;
     }
 
+    *ttcIndex = 0;
     return stream;
 }