Initial linear gradient 4f impl

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1688543002

Review URL: https://codereview.chromium.org/1688543002
diff --git a/src/effects/gradients/SkLinearGradient.h b/src/effects/gradients/SkLinearGradient.h
index c8b91a5..c81eea6 100644
--- a/src/effects/gradients/SkLinearGradient.h
+++ b/src/effects/gradients/SkLinearGradient.h
@@ -26,6 +26,11 @@
 
 class SkLinearGradient : public SkGradientShaderBase {
 public:
+    enum {
+        // Temp flag for testing the 4f impl.
+        kForce4fContext_PrivateFlag     = 1 << 7,
+    };
+
     SkLinearGradient(const SkPoint pts[2], const Descriptor&);
 
     size_t contextSize() const override;
@@ -69,6 +74,8 @@
     Context* onCreateContext(const ContextRec&, void* storage) const override;
 
 private:
+    class LinearGradient4fContext;
+
     friend class SkGradientShader;
     typedef SkGradientShaderBase INHERITED;
     const SkPoint fStart;