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/src/gpu/effects/GrPorterDuffXferProcessor.cpp b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
index d511fae..ff95dac 100644
--- a/src/gpu/effects/GrPorterDuffXferProcessor.cpp
+++ b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
@@ -227,7 +227,7 @@
  * with and without an opaque input color. Optimization properties are deduced at compile time so we
  * can make runtime decisions quickly. RGB coverage is not supported.
  */
-static const BlendFormula gBlendTable[2][2][SkXfermode::kLastCoeffMode + 1] = {
+static const BlendFormula gBlendTable[2][2][(int)SkBlendMode::kLastCoeffMode + 1] = {
 
                      /*>> No coverage, input color unknown <<*/ {{
 
@@ -302,7 +302,7 @@
     /* screen */     COEFF_FORMULA(   kOne_GrBlendCoeff,    kISC_GrBlendCoeff),
 }}};
 
-static const BlendFormula gLCDBlendTable[SkXfermode::kLastCoeffMode + 1] = {
+static const BlendFormula gLCDBlendTable[(int)SkBlendMode::kLastCoeffMode + 1] = {
     /* clear */      COVERAGE_SRC_COEFF_ZERO_FORMULA(BlendFormula::kCoverage_OutputType),
     /* src */        COVERAGE_FORMULA(BlendFormula::kCoverage_OutputType, kOne_GrBlendCoeff),
     /* dst */        NO_DST_WRITE_FORMULA,