Use stencil ops that don't issue writes whenever possible

src/gpu/GrStencilAndCoverTextContext.cpp

Index: src/gpu/GrStencilAndCoverTextContext.cpp
diff --git a/src/gpu/GrStencilAndCoverTextContext.cpp b/src/gpu/GrStencilAndCoverTextContext.cpp
index 980a494eafd01bc2ac55c4177459fc7ce80c2fc1..554038fc32fe330db5f6262247ea0b53fd313786 100644
--- a/src/gpu/GrStencilAndCoverTextContext.cpp
+++ b/src/gpu/GrStencilAndCoverTextContext.cpp
@@ -435,7 +435,7 @@ void GrStencilAndCoverTextContext::flush() {
 
         GR_STATIC_CONST_SAME_STENCIL(kStencilPass,
                                      kZero_StencilOp,
-                                     kZero_StencilOp,
+                                     kKeep_StencilOp,
                                      kNotEqual_StencilFunc,
                                      0xffff,
                                      0x0000,