Make onPrepareDraws const

src/gpu/batches/GrAAStrokeRectBatch.cpp

Index: src/gpu/batches/GrAAStrokeRectBatch.cpp
diff --git a/src/gpu/batches/GrAAStrokeRectBatch.cpp b/src/gpu/batches/GrAAStrokeRectBatch.cpp
index 9addc2fa417b9f4d6a45424932bd34ea562ba09d..e729ae2404eb4da8fd0821bd46e6c4fa77d6931e 100644
--- a/src/gpu/batches/GrAAStrokeRectBatch.cpp
+++ b/src/gpu/batches/GrAAStrokeRectBatch.cpp
@@ -107,7 +107,7 @@ private:
         bounds->join(geo.fDevOutsideAssist);
     }
 
-    void onPrepareDraws(Target*) override;
+    void onPrepareDraws(Target*) const override;
     void initBatchTracker(const GrPipelineOptimizations&) override;
 
     AAStrokeRectBatch(const SkMatrix& viewMatrix,bool miterStroke)
@@ -182,7 +182,7 @@ void AAStrokeRectBatch::initBatchTracker(const GrPipelineOptimizations& opt) {
     fBatch.fCanTweakAlphaForCoverage = opt.canTweakAlphaForCoverage();
 }
 
-void AAStrokeRectBatch::onPrepareDraws(Target* target) {
+void AAStrokeRectBatch::onPrepareDraws(Target* target) const {
     bool canTweakAlphaForCoverage = this->canTweakAlphaForCoverage();
 
     SkAutoTUnref<const GrGeometryProcessor> gp(create_stroke_rect_gp(canTweakAlphaForCoverage,