SkDocument:  Factories now located in SkPDFDocument.h and SkXPSDocument.h

Change-Id: I48e73b27e52511292c2c0bd51ef0168766f53a80
Reviewed-on: https://skia-review.googlesource.com/152780
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Reed <reed@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index ef2fd47..cb845bc 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -93,6 +93,7 @@
   "include/codec",
   "include/config",
   "include/core",
+  "include/docs",
   "include/effects",
   "include/encode",
   "include/gpu",