Make XPFragmentBuilder only Builder with access to DstCopy.

src/gpu/gl/builders/GrGLProgramBuilder.h

Index: src/gpu/gl/builders/GrGLProgramBuilder.h
diff --git a/src/gpu/gl/builders/GrGLProgramBuilder.h b/src/gpu/gl/builders/GrGLProgramBuilder.h
index 2e9ab863d2fa538e16cc904057e27034e0aa5755..e7520eee10edff044b9f15754d700e150afbc75d 100644
--- a/src/gpu/gl/builders/GrGLProgramBuilder.h
+++ b/src/gpu/gl/builders/GrGLProgramBuilder.h
@@ -155,7 +155,7 @@ public:
                                          const char* output) = 0;
 
     // TODO rename getFragmentBuilder
-    virtual GrGLGPFragmentBuilder* getFragmentShaderBuilder() = 0;
+    virtual GrGLFragmentBuilder* getFragmentShaderBuilder() = 0;
     virtual GrGLVertexBuilder* getVertexShaderBuilder() = 0;
 
     /*
@@ -166,7 +166,7 @@ public:
 /* a specializations for FPs. Lets the user add uniforms and FS code */
 class GrGLFPBuilder : public virtual GrGLUniformBuilder {
 public:
-    virtual GrGLFPFragmentBuilder* getFragmentShaderBuilder() = 0;
+    virtual GrGLFragmentBuilder* getFragmentShaderBuilder() = 0;
 
     /*
      * *NOTE* NO MEMBERS ALLOWED, MULTIPLE INHERITANCE
@@ -176,7 +176,7 @@ public:
 /* a specializations for XPs. Lets the user add uniforms and FS code */
 class GrGLXPBuilder : public virtual GrGLUniformBuilder {
 public:
-    virtual GrGLFPFragmentBuilder* getFragmentShaderBuilder() = 0;
+    virtual GrGLXPFragmentBuilder* getFragmentShaderBuilder() = 0;
 
     /*
      * *NOTE* NO MEMBERS ALLOWED, MULTIPLE INHERITANCE
@@ -248,7 +248,7 @@ public:
 
     GrGLGpu* gpu() const override { return fGpu; }
 
-    GrGLFPFragmentBuilder* getFragmentShaderBuilder() override { return &fFS; }
+    GrGLXPFragmentBuilder* getFragmentShaderBuilder() override { return &fFS; }
     GrGLVertexBuilder* getVertexShaderBuilder() override { return &fVS; }
 
     void addVarying(