Start to propagate constness of GrPipelineBuilder up the stack

src/gpu/gl/GrGLGpu.h

Index: src/gpu/gl/GrGLGpu.h
diff --git a/src/gpu/gl/GrGLGpu.h b/src/gpu/gl/GrGLGpu.h
index 368bda17974da63ff36417d761ca713ecb336352..baafea3f1ee97e00ef29ca263741b850c9d890e7 100644
--- a/src/gpu/gl/GrGLGpu.h
+++ b/src/gpu/gl/GrGLGpu.h
@@ -70,7 +70,7 @@ public:
                                    size_t rowBytes) const override;
     bool fullReadPixelsIsFasterThanPartial() const override;
 
-    bool initCopySurfaceDstDesc(const GrSurface* src, GrSurfaceDesc* desc) override;
+    bool initCopySurfaceDstDesc(const GrSurface* src, GrSurfaceDesc* desc) const override;
 
     // These functions should be used to bind GL objects. They track the GL state and skip redundant
     // bindings. Making the equivalent glBind calls directly will confuse the state tracking.