Move some work from backend onClear to base class clear

src/gpu/GrDrawTarget.h

Index: src/gpu/GrDrawTarget.h
diff --git a/src/gpu/GrDrawTarget.h b/src/gpu/GrDrawTarget.h
index 022c6b3e2d25b77a6e9de418bb79f4fec6b3ec4f..a38f0e9cb029a7a3aff5019235c6ec9eb6a9510a 100644
--- a/src/gpu/GrDrawTarget.h
+++ b/src/gpu/GrDrawTarget.h
@@ -271,8 +271,7 @@ private:
                              const GrStencilSettings&,
                              const PipelineInfo&) = 0;
 
-    virtual void onClear(const SkIRect* rect, GrColor color, bool canIgnoreRect,
-                         GrRenderTarget* renderTarget) = 0;
+    virtual void onClear(const SkIRect& rect, GrColor color, GrRenderTarget* renderTarget) = 0;
 
     /** The subclass's copy surface implementation. It should assume that any clipping has already
         been performed on the rect and point and that the GrGpu supports the copy. */