Don't allow renderTarget==NULL to GrContext::clear() and friends.

src/gpu/GrDrawTarget.h

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrDrawTarget_DEFINED
 #define GrDrawTarget_DEFINED
 
@@ skipping 392 matching lines @@
      * @param devBounds    optional bounds hint. This is a promise from the caller,
      *                     not a request for clipping.
      */
     void drawIndexedInstances(GrPrimitiveType type,
                               int instanceCount,
                               int verticesPerInstance,
                               int indicesPerInstance,
                               const SkRect* devBounds = NULL);
 
     /**
-     * Clear the current render target if one isn't passed in. Ignores the
-     * clip and all other draw state (blend mode, stages, etc). Clears the
-     * whole thing if rect is NULL, otherwise just the rect. If canIgnoreRect
-     * is set then the entire render target can be optionally cleared.
+     * Clear the passed in render target. Ignores the draw state and clip. Clears the whole thing if
+     * rect is NULL, otherwise just the rect. If canIgnoreRect is set then the entire render target
+     * can be optionally cleared.
      */
-    virtual void clear(const SkIRect* rect,
-                       GrColor color,
-                       bool canIgnoreRect,
-                       GrRenderTarget* renderTarget = NULL) = 0;
+    virtual void clear(const SkIRect* rect, GrColor color, bool canIgnoreRect,
+                       GrRenderTarget* renderTarget) = 0;
 
     /**
-     * Discards the contents render target. NULL indicates that the current render target should
-     * be discarded.
+     * Discards the contents render target.
      **/
-    virtual void discard(GrRenderTarget* = NULL) = 0;
+    virtual void discard(GrRenderTarget*) = 0;
 
     /**
      * Called at start and end of gpu trace marking
      * GR_CREATE_GPU_TRACE_MARKER(marker_str, target) will automatically call these at the start
      * and end of a code block respectively
      */
     void addGpuTraceMarker(const GrGpuTraceMarker* marker);
     void removeGpuTraceMarker(const GrGpuTraceMarker* marker);
 
     /**
@@ skipping 542 matching lines @@
 protected:
     GrClipMaskManager           fClipMaskManager;
 
 private:
     GrClipMaskManager* getClipMaskManager() { return &fClipMaskManager; }
 
     typedef GrDrawTarget INHERITED;
 };
 
 #endif