Simplify flattening to just write enough to call the factory

src/effects/gradients/SkGradientShader.cpp

Index: src/effects/gradients/SkGradientShader.cpp
diff --git a/src/effects/gradients/SkGradientShader.cpp b/src/effects/gradients/SkGradientShader.cpp
index fb8a7e5c8f04d1414d7443fd367547267ac846d6..37374662a56ba146496579fa955e0589a8852252 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -139,6 +139,7 @@ static uint32_t unpack_flags(uint32_t packed) {
     return packed >> 4;
 }
 
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
 SkGradientShaderBase::SkGradientShaderBase(SkReadBuffer& buffer) : INHERITED(buffer) {
     if (buffer.isVersionLT(SkReadBuffer::kNoUnitMappers_Version)) {
         // skip the old SkUnitMapper slot
@@ -177,6 +178,7 @@ SkGradientShaderBase::SkGradientShaderBase(SkReadBuffer& buffer) : INHERITED(buf
     buffer.readMatrix(&fPtsToUnit);
     this->initCommon();
 }
+#endif
 
 SkGradientShaderBase::~SkGradientShaderBase() {
     if (fOrigColors != fStorage) {