Put clear and discard into GrBatch.

src/gpu/batches/GrDiscardBatch.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef GrDiscardBatch_DEFINED
+#define GrDiscardBatch_DEFINED
+
+#include "GrBatch.h"
+#include "GrBatchFlushState.h"
+#include "GrGpu.h"
+#include "GrRenderTarget.h"
+
+class GrDiscardBatch final : public GrBatch {
+public:
+    GrDiscardBatch(GrRenderTarget* rt)
+        : fRenderTarget(rt) {
+        this->initClassID<GrDiscardBatch>();
+        fBounds = SkRect::MakeWH(SkIntToScalar(rt->width()), SkIntToScalar(rt->height()));
+    }
+
+    const char* name() const override { return "Discard"; }
+
+    uint32_t renderTargetUniqueID() const override { return fRenderTarget.get()->getUniqueID(); }
+
+    SkString dumpInfo() const {
+        SkString string;
+        string.printf("RT: 0x%p", fRenderTarget.get());
+        return string;
+    }
+
+private:
+    bool onCombineIfPossible(GrBatch* that, const GrCaps& caps) override {
+        return fRenderTarget == that->cast<GrDiscardBatch>()->fRenderTarget;
+    }
+
+    void onPrepare(GrBatchFlushState*) override {}
+
+    void onDraw(GrBatchFlushState* state) override {
+        state->gpu()->discard(fRenderTarget.get());
+    }
+
+    GrPendingIOResource<GrRenderTarget, kWrite_GrIOType> fRenderTarget;
+};
+
+#endif