Change visitProxies signature

There is a perf regression (mainly on the Nexus5) for the https://skia-review.googlesource.com/c/skia/+/46200 (Add method to iterate over a GrOp's GrSurfaceProxies)

This is one candidate.

Change-Id: I995d3a88bad2a914f24b49512abbf01aeaf579c8
Reviewed-on: https://skia-review.googlesource.com/46586
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/gm/convexpolyeffect.cpp b/gm/convexpolyeffect.cpp
index ab9a4d0..a4fb5e5 100644
--- a/gm/convexpolyeffect.cpp
+++ b/gm/convexpolyeffect.cpp
@@ -48,7 +48,7 @@
 
     const char* name() const override { return "PolyBoundsOp"; }
 
-    void visitProxies(VisitProxyFunc func) const override {
+    void visitProxies(const VisitProxyFunc& func) const override {
         fProcessors.visitProxies(func);
     }