DM: don't leak the null canvas in NullSink.

dm/DMSrcSink.h

 #ifndef DMSrcSink_DEFINED
 #define DMSrcSink_DEFINED
 
 #include "DMGpuSupport.h"
 #include "SkBBHFactory.h"
 #include "SkBBoxHierarchy.h"
 #include "SkBitmap.h"
 #include "SkCanvas.h"
 #include "SkData.h"
 #include "SkGPipe.h"
-#include "SkNullCanvas.h"
 #include "SkPicture.h"
 #include "gm.h"
 
 namespace DM {
 
 // This is just convenience.  It lets you use either return "foo" or return SkStringPrintf(...).
 struct ImplicitString : public SkString {
     template <typename T>
     ImplicitString(const T& s) : SkString(s) {}
 };
@@ skipping 57 matching lines @@
 
     Error draw(SkCanvas*) const SK_OVERRIDE;
     SkISize size() const SK_OVERRIDE;
     Name name() const SK_OVERRIDE;
 private:
     Path fPath;
 };
 
 /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
 
+class NullSink : public Sink {
+public:
+    NullSink() {}
+
+    Error draw(const Src& src, SkBitmap*, SkWStream*, SkString*) const SK_OVERRIDE;
+    int enclave() const SK_OVERRIDE { return kAnyThread_Enclave; }
+    const char* fileExtension() const SK_OVERRIDE { return ""; }
+};
+
+
 class GPUSink : public Sink {
 public:
     GPUSink(GrContextFactory::GLContextType, GrGLStandard, int samples, bool dfText, bool threaded);
 
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const SK_OVERRIDE;
     int enclave() const SK_OVERRIDE;
     const char* fileExtension() const SK_OVERRIDE { return "png"; }
 private:
     GrContextFactory::GLContextType fContextType;
     GrGLStandard                    fGpuAPI;
@@ skipping 83 matching lines @@
 
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const SK_OVERRIDE;
     int enclave() const SK_OVERRIDE { return fSink->enclave(); }
     const char* fileExtension() const SK_OVERRIDE { return fSink->fileExtension(); }
 private:
     const int                   fW, fH;
     SkAutoTDelete<SkBBHFactory> fFactory;
     SkAutoTDelete<Sink>         fSink;
 };
 
-class NullSink : public Sink {
-public:
-    Error draw(const Src& src, SkBitmap*, SkWStream*, SkString*) const SK_OVERRIDE {
-        return src.draw(SkCreateNullCanvas());
-    }
-    int enclave() const SK_OVERRIDE { return kAnyThread_Enclave; }
-    const char* fileExtension() const SK_OVERRIDE { return ""; }
-};
-
 }  // namespace DM
 
 #endif//DMSrcSink_DEFINED