Remove include/xml from public API.

Move SkDOM and ilk from include/xml to src/xml.

I have looked for uses of SkDOM in the depot using cs/ and have found none,
but this is not perfect. So, if this breaks external builds, revert it.

TBR=reed@google.com

Change-Id: I162a1977f0649b049c0f93f016701784d025996d
Reviewed-on: https://skia-review.googlesource.com/8447
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Herb Derby <herb@google.com>

BUILD.gn

@@ -67,7 +67,6 @@ skia_public_includes = [
   "include/svg",
   "include/utils",
   "include/utils/mac",
-  "include/xml",
 ]
 
 # Skia public API, generally provided by :skia.
@@ -112,6 +111,7 @@ config("skia_private") {
     "src/sksl",
     "src/utils",
     "src/utils/win",
+    "src/xml",
     "third_party/etc1",
     "third_party/gif",
     "third_party/ktx",

src/xml/SkDOM.h

@@ -20,7 +20,7 @@ class SkDOMParser;
 class SkStream;
 class SkXMLParser;
 
-class SK_API SkDOM : public SkNoncopyable {
+class SkDOM : public SkNoncopyable {
 public:
     SkDOM();
     ~SkDOM();