Don't use NULL GrOptDrawState to indicate that draw should be skipped.

include/gpu/GrGpuResourceRef.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGpuResourceRef_DEFINED
 #define GrGpuResourceRef_DEFINED
 
@@ skipping 168 matching lines @@
             }
         }
         this->release();
         fResource = resource;
     }
 
     ~GrPendingIOResource() {
         this->release();
     }
 
+    operator bool() const { return SkToBool(fResource); }
+
     T* get() const { return fResource; }
 
 private:
     void release() {
         if (fResource) {
             switch (IO_TYPE) {
                 case kRead_GrIOType:
                     fResource->completedRead();
                     break;
                 case kWrite_GrIOType:
                     fResource->completedWrite();
                     break;
                 case kRW_GrIOType:
                     fResource->completedRead();
                     fResource->completedWrite();
                     break;
             }
         }
     }
 
     T* fResource;
 };
 #endif