Put clear and discard into GrBatch.

src/gpu/batches/GrClearBatch.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 GrClearBatch_DEFINED
+#define GrClearBatch_DEFINED
+
+#include "GrBatch.h"
+#include "GrBatchFlushState.h"
+#include "GrGpu.h"
+#include "GrRenderTarget.h"
+
+class GrClearBatch final : public GrBatch {
+public:
+    GrClearBatch(const SkIRect& rect,  GrColor color, GrRenderTarget* rt)
+        : fRect(rect)
+        , fColor(color)
+        , fRenderTarget(rt) {
+        this->initClassID<GrClearBatch>();
+        fBounds = SkRect::Make(rect);
+    }
+
+    const char* name() const override { return "Clear"; }
+
+    uint32_t renderTargetUniqueID() const override { return fRenderTarget.get()->getUniqueID(); }
+
+    SkString dumpInfo() const {
+        SkString string;
+        string.printf("Color: 0x%08x, Rect [L: %d, T: %d, R: %d, B: %d], RT: 0x%p",
+                      fColor, fRect.fLeft, fRect.fTop, fRect.fRight, fRect.fBottom,
+                      fRenderTarget.get());
+        return string;
+    }
+
+private:
+    bool onCombineIfPossible(GrBatch* t, const GrCaps& caps) override {
+        // We could combine clears. TBD how much complexity to put here.
+        return false;
+    }
+
+    void onPrepare(GrBatchFlushState*) override {}
+
+    void onDraw(GrBatchFlushState* state) override {
+        state->gpu()->clear(fRect, fColor, fRenderTarget.get());
+    }
+
+    SkIRect                                                 fRect;
+    GrColor                                                 fColor;
+    GrPendingIOResource<GrRenderTarget, kWrite_GrIOType>    fRenderTarget;
+};
+
+#endif