Move some work from backend onClear to base class clear

src/gpu/GrTargetCommands.cpp

Index: src/gpu/GrTargetCommands.cpp
diff --git a/src/gpu/GrTargetCommands.cpp b/src/gpu/GrTargetCommands.cpp
index 3096e27692cb6b3b9810beae1e4d1da9357de53b..713514fab89f3c55e80184d6da6ec9d626a209e2 100644
--- a/src/gpu/GrTargetCommands.cpp
+++ b/src/gpu/GrTargetCommands.cpp
@@ -91,7 +91,7 @@ void GrTargetCommands::Clear::execute(GrGpu* gpu) {
     if (GrColor_ILLEGAL == fColor) {
         gpu->discard(this->renderTarget());
     } else {
-        gpu->clear(&fRect, fColor, fCanIgnoreRect, this->renderTarget());
+        gpu->clear(fRect, fColor, this->renderTarget());
     }
 }