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

src/gpu/GrGpu.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGpu_DEFINED
 #define GrGpu_DEFINED
 
@@ skipping 248 matching lines @@
      * @param buffer        memory to read pixels from
      * @param rowBytes      number of bytes between consecutive rows. Zero
      *                      means rows are tightly packed.
      */
     bool writeTexturePixels(GrTexture* texture,
                             int left, int top, int width, int height,
                             GrPixelConfig config, const void* buffer,
                             size_t rowBytes);
 
     // GrDrawTarget overrides
-    virtual void clear(const SkIRect* rect,
-                       GrColor color,
-                       bool canIgnoreRect,
-                       GrRenderTarget* renderTarget = NULL) SK_OVERRIDE;
+    virtual void clear(const SkIRect* rect, GrColor color, bool canIgnoreRect,
+                       GrRenderTarget* renderTarget) SK_OVERRIDE;
 
     virtual void clearStencilClip(const SkIRect& rect,
                                   bool insideClip,
                                   GrRenderTarget* renderTarget = NULL) SK_OVERRIDE;
 
     // After the client interacts directly with the 3D context state the GrGpu
     // must resync its internal state and assumptions about 3D context state.
     // Each time this occurs the GrGpu bumps a timestamp.
     // state of the 3D context
     // At 10 resets / frame and 60fps a 64bit timestamp will overflow in about
@@ skipping 205 matching lines @@
     // counts number of uses of vertex/index pool in the geometry stack
     int                                                                 fVertexPoolUseCnt;
     int                                                                 fIndexPoolUseCnt;
     // these are mutable so they can be created on-demand
     mutable GrIndexBuffer*                                              fQuadIndexBuffer;
 
     typedef GrClipTarget INHERITED;
 };
 
 #endif