make SkXfermode.h go away

This is step one:
- make SkXfermode useless to public clients
- everything they should need is in SkBlendMode.h

Step two:
- remove SkXfermode.h entirely (since skia core will already be using SkXfermodePriv.h)

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4534

Change-Id: If2cea9f71df92430ed6644edb98dd306c5572cbc
Reviewed-on: https://skia-review.googlesource.com/4534
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
diff --git a/tests/RecordingXfermodeTest.cpp b/tests/RecordingXfermodeTest.cpp
index 32aec36..db4262c 100644
--- a/tests/RecordingXfermodeTest.cpp
+++ b/tests/RecordingXfermodeTest.cpp
@@ -12,6 +12,7 @@
 #include "../include/core/SkStream.h"
 #include "../include/core/SkString.h"
 #include "../include/core/SkPictureRecorder.h"
+#include "../src/core/SkBlendModePriv.h"
 #include <cstring>
 
 // Verify that replay of a recording into a clipped canvas
@@ -160,7 +161,7 @@
         if (memcmp(goldenBM.getPixels(), pictureBM.getPixels(), pixelsSize)) {
             numErrors++;
             errors.appendf("For SkXfermode %d %s:    SkPictureRecorder bitmap is wrong\n",
-                           iMode, SkXfermode::ModeName(mode));
+                           iMode, SkBlendMode_Name(mode));
         }
 #endif
     }