Loosen requirement that there be only one GrDrawTarget

include/gpu/GrDrawContext.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 GrDrawContext_DEFINED
 #define GrDrawContext_DEFINED
 
@@ skipping 238 matching lines @@
      * @param batch    the batch to draw
      */
     void drawBatch(const GrClip&, const GrPaint&, GrDrawBatch*);
 
 private:
     friend class GrAtlasTextContext; // for access to drawBatch
     friend class GrContext; // for ctor
 
     SkDEBUGCODE(void validate() const;)
 
-    GrDrawContext(GrContext*, GrRenderTarget*, GrDrawTarget*, const SkSurfaceProps* surfaceProps);
+    GrDrawContext(GrContext*, GrRenderTarget*, const SkSurfaceProps* surfaceProps);
 
     void internalDrawPath(GrDrawTarget*,
                           GrPipelineBuilder*,
                           const SkMatrix& viewMatrix,
                           GrColor,
                           bool useAA,
                           const SkPath&,
                           const GrStrokeInfo&);
 
     // This entry point allows the GrTextContext-derived classes to add their batches to
     // the drawTarget.
     void drawBatch(GrPipelineBuilder* pipelineBuilder, GrDrawBatch* batch);
 
+    GrDrawTarget* getDrawTarget();
+
     GrContext*          fContext;     // owning context -> no ref
     GrRenderTarget*     fRenderTarget;
+
+    // In MDB-mode the drawTarget can be closed by some other drawContext that has picked
+    // it up. For this reason, the drawTarget should only ever be accessed via 'getDrawTarget'.
     GrDrawTarget*       fDrawTarget;
     GrTextContext*      fTextContext; // lazily gotten from GrContext::DrawingMgr
 
     SkSurfaceProps      fSurfaceProps;
 };
 
 #endif