Start to propagate constness of GrPipelineBuilder up the stack

BUG=skia:

Review URL: https://codereview.chromium.org/1229143007
diff --git a/src/gpu/GrTessellatingPathRenderer.cpp b/src/gpu/GrTessellatingPathRenderer.cpp
index de5dcab..f420372 100644
--- a/src/gpu/GrTessellatingPathRenderer.cpp
+++ b/src/gpu/GrTessellatingPathRenderer.cpp
@@ -1511,7 +1511,7 @@
     }
     vmi.mapRect(&clipBounds);
     SkAutoTUnref<GrBatch> batch(TessellatingPathBatch::Create(color, path, viewM, clipBounds));
-    target->drawBatch(pipelineBuilder, batch);
+    target->drawBatch(*pipelineBuilder, batch);
 
     return true;
 }