Move some work from backend onClear to base class clear

src/gpu/GrTargetCommands.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 GrTargetCommands_DEFINED
 #define GrTargetCommands_DEFINED
 
@@ skipping 165 matching lines @@
     // This is also used to record a discard by setting the color to GrColor_ILLEGAL
     struct Clear : public Cmd {
         Clear(GrRenderTarget* rt) : Cmd(kClear_CmdType), fRenderTarget(rt) {}
 
         GrRenderTarget* renderTarget() const { return fRenderTarget.get(); }
 
         void execute(GrGpu*) override;
 
         SkIRect fRect;
         GrColor fColor;
-        bool    fCanIgnoreRect;
 
     private:
         GrPendingIOResource<GrRenderTarget, kWrite_GrIOType> fRenderTarget;
     };
 
     // This command is ONLY used by the clip mask manager to clear the stencil clip bits
     struct ClearStencilClip : public Cmd {
         ClearStencilClip(GrRenderTarget* rt) : Cmd(kClearStencil_CmdType), fRenderTarget(rt) {}
 
         GrRenderTarget* renderTarget() const { return fRenderTarget.get(); }
@@ skipping 66 matching lines @@
 
     CmdBuffer* cmdBuffer() { return &fCmdBuffer; }
     GrBatchTarget* batchTarget() { return &fBatchTarget; }
 
     CmdBuffer                           fCmdBuffer;
     GrBatchTarget                       fBatchTarget;
 };
 
 #endif