Move some work from backend onClear to base class clear

src/gpu/GrImmediateDrawTarget.cpp

Index: src/gpu/GrImmediateDrawTarget.cpp
diff --git a/src/gpu/GrImmediateDrawTarget.cpp b/src/gpu/GrImmediateDrawTarget.cpp
index 0099b1dc8cab705582a694af5609b31a8ee378e8..c75f23f9b72cb7338e41ea9af68f8eac5226590f 100644
--- a/src/gpu/GrImmediateDrawTarget.cpp
+++ b/src/gpu/GrImmediateDrawTarget.cpp
@@ -50,8 +50,8 @@ void GrImmediateDrawTarget::onDrawBatch(GrBatch* batch,
 }
 
 void GrImmediateDrawTarget::onClear(const SkIRect* rect, GrColor color,
-                                    bool canIgnoreRect, GrRenderTarget* renderTarget) {
-    this->getGpu()->clear(rect, color, canIgnoreRect, renderTarget);
+                                    GrRenderTarget* renderTarget) {
+    this->getGpu()->clear(rect, color, renderTarget);
 }
 
 void GrImmediateDrawTarget::onCopySurface(GrSurface* dst,