Use SkXfermode as public facing enum for GrPorterDuffXP

src/gpu/SkGpuDevice.cpp

Index: src/gpu/SkGpuDevice.cpp
diff --git a/src/gpu/SkGpuDevice.cpp b/src/gpu/SkGpuDevice.cpp
index 8912851d8f369c88a9b36aeb7d281c8b2490c690..70d8dfc836cca8d316abf85e2403256ab1a57611 100644
--- a/src/gpu/SkGpuDevice.cpp
+++ b/src/gpu/SkGpuDevice.cpp
@@ -9,7 +9,6 @@
 
 #include "effects/GrBicubicEffect.h"
 #include "effects/GrDashingEffect.h"
-#include "effects/GrPorterDuffXferProcessor.h"
 #include "effects/GrTextureDomain.h"
 #include "effects/GrSimpleTextureEffect.h"
 
@@ -679,13 +678,7 @@ GrTexture* create_mask_GPU(GrContext* context,
     GrPaint tempPaint;
     if (doAA) {
         tempPaint.setAntiAlias(true);
-        // AA uses the "coverage" stages on GrDrawTarget. Coverage with a dst
-        // blend coeff of zero requires dual source blending support in order
-        // to properly blend partially covered pixels. This means the AA
-        // code path may not be taken. So we use a dst blend coeff of ISA. We
-        // could special case AA draws to a dst surface with known alpha=0 to
-        // use a zero dst coeff when dual source blending isn't available.
-        tempPaint.setPorterDuffXPFactory(kOne_GrBlendCoeff, kISC_GrBlendCoeff);
+        tempPaint.setCoverageSetOpXPFactory(SkRegion::kUnion_Op);

[bsalomon, 2015/02/14 20:45:59]

I know we spent time figuring out that this is union, but now I realize that the
old code was trying to draw with replace, but to avoid triggering dsb/dst-copy
it was using union. Since the dst was cleared to zero union is equivalent to
replace. Might as well use replace.

[bsalomon, 2015/02/17 13:58:13]

Also, we should install this XP unconditionally. It was guarded by doAA only
because this code had to try to be too smart in the pre-XP days by knowing that
aa could trigger coverage stages and therefore present blending challenges in
src mode.

[egdaniel, 2015/02/17 14:43:01]

Do we still want to set aa on the paint? Should it be tempPaint.setAA(doAA)?

     }
 
     // Draw the mask into maskTexture with the path's top-left at the origin using tempPaint.