Add onGetFamilyName to SkTypeface.

This speeds up and documents this particular feature of SkTypeface
and also frees up SkFontDescriptor to be used only in serialization.

R=reed@google.com

Author: bungeman@google.com

Review URL: https://codereview.chromium.org/574873002
diff --git a/src/fonts/SkGScalerContext.cpp b/src/fonts/SkGScalerContext.cpp
index e543fdb..34a788a 100644
--- a/src/fonts/SkGScalerContext.cpp
+++ b/src/fonts/SkGScalerContext.cpp
@@ -206,6 +206,10 @@
     return fProxy->getUnitsPerEm();
 }
 
+void SkGTypeface::onGetFamilyName(SkString* familyName) const {
+    fProxy->getFamilyName(familyName);
+}
+
 SkTypeface::LocalizedStrings* SkGTypeface::onCreateFamilyNameIterator() const {
     return fProxy->createFamilyNameIterator();
 }