Move some work from backend onClear to base class clear

include/gpu/GrCaps.h

Index: include/gpu/GrCaps.h
diff --git a/include/gpu/GrCaps.h b/include/gpu/GrCaps.h
index 5452eb329f87b406e06775fe07de638d21b24fd7..c9530d3720457f2890020d0b373476bf8f907013 100644
--- a/include/gpu/GrCaps.h
+++ b/include/gpu/GrCaps.h
@@ -216,6 +216,8 @@ public:
         return fSupportsInstancedDraws;
     }
 
+    bool fullClearIsFree() const { return fFullClearIsFree; }
+
 protected:
     /** Subclasses must call this at the end of their constructors in order to apply caps
         overrides requested by the client. Note that overrides will only reduce the caps never
@@ -236,6 +238,7 @@ protected:
     bool fOversizedStencilSupport                    : 1;
     bool fTextureBarrierSupport                      : 1;
     bool fSupportsInstancedDraws                     : 1;
+    bool fFullClearIsFree                            : 1;
 
     // Driver workaround
     bool fUseDrawInsteadOfClear                      : 1;