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/bench/RotatedRectBench.cpp b/bench/RotatedRectBench.cpp
index f81bfb1..d9ee582 100644
--- a/bench/RotatedRectBench.cpp
+++ b/bench/RotatedRectBench.cpp
@@ -6,6 +6,7 @@
  */
 
 #include "Benchmark.h"
+#include "SkBlendModePriv.h"
 #include "SkCanvas.h"
 #include "SkPaint.h"
 
@@ -144,7 +145,7 @@
                 fName.append("_alternating_transparent_and_opaque");
                 break;
         }
-        fName.appendf("_%s", to_lower(SkXfermode::ModeName(fMode)).c_str());
+        fName.appendf("_%s", to_lower(SkBlendMode_Name(fMode)).c_str());
     }
 
     bool        fAA;