Start to propagate constness of GrPipelineBuilder up the stack

BUG=skia:

Review URL: https://codereview.chromium.org/1229143007
diff --git a/src/gpu/GrSWMaskHelper.cpp b/src/gpu/GrSWMaskHelper.cpp
index 28071d2..0e0a541 100644
--- a/src/gpu/GrSWMaskHelper.cpp
+++ b/src/gpu/GrSWMaskHelper.cpp
@@ -374,5 +374,5 @@
                                                        GrTextureParams::kNone_FilterMode,
                                                        kDevice_GrCoordSet))->unref();
 
-    target->drawBWRect(pipelineBuilder, color, SkMatrix::I(), dstRect, NULL, &invert);
+    target->drawBWRect(*pipelineBuilder, color, SkMatrix::I(), dstRect, NULL, &invert);
 }