baby steps to GN check correctness

re-export optional parts of :skia

Now passing,
    $ gn gen out; gn check out :dm

And we think Flutter too.

Change-Id: I4886c484711cce0c18b24828dc659fbf541a8b87
Reviewed-on: https://skia-review.googlesource.com/c/169343
Reviewed-by: Mehmet Fidanboylu <mehmetf@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index f76d918..d62b0bd 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -859,27 +859,30 @@
   public_configs = [ ":skia_public" ]
   configs += skia_library_configs
 
+  public_deps = [
+    ":effects",
+    ":gpu",
+    ":pdf",
+    ":skcms",
+  ]
+
   deps = [
     ":arm64",
     ":armv7",
     ":avx",
     ":crc32",
     ":effect_deserialization",
-    ":effects",
     ":fontmgr_android",
     ":fontmgr_custom",
     ":fontmgr_empty",
     ":fontmgr_fontconfig",
     ":fontmgr_fuchsia",
-    ":gpu",
     ":heif",
     ":hsw",
     ":jpeg",
     ":none",
-    ":pdf",
     ":png",
     ":raw",
-    ":skcms",
     ":sse2",
     ":sse41",
     ":sse42",