Make XPFragmentBuilder only Builder with access to DstCopy.
Plus a bunch of renaming.

BUG=skia:

Review URL: https://codereview.chromium.org/1110033004
diff --git a/src/gpu/effects/GrSimpleTextureEffect.cpp b/src/gpu/effects/GrSimpleTextureEffect.cpp
index 2fe6dfe..e729b1b 100644
--- a/src/gpu/effects/GrSimpleTextureEffect.cpp
+++ b/src/gpu/effects/GrSimpleTextureEffect.cpp
@@ -24,7 +24,7 @@
                           const char* inputColor,
                           const TransformedCoordsArray& coords,
                           const TextureSamplerArray& samplers) override {
-        GrGLFPFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
+        GrGLFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
         fsBuilder->codeAppendf("\t%s = ", outputColor);
         fsBuilder->appendTextureLookupAndModulate(inputColor,
                                                   samplers[0],