Add SkColor4f serialization

Adjusted usage in color shader, and will also be using this
in gradients, soon.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2334123003

Review-Url: https://codereview.chromium.org/2334123003
diff --git a/tools/debugger/SkDrawCommand.h b/tools/debugger/SkDrawCommand.h
index 27b56e2..df50be8 100644
--- a/tools/debugger/SkDrawCommand.h
+++ b/tools/debugger/SkDrawCommand.h
@@ -123,6 +123,7 @@
 
     // Helper methods for converting things to JSON
     static Json::Value MakeJsonColor(const SkColor color);
+    static Json::Value MakeJsonColor4f(const SkColor4f& color);
     static Json::Value MakeJsonPoint(const SkPoint& point);
     static Json::Value MakeJsonPoint(SkScalar x, SkScalar y);
     static Json::Value MakeJsonRect(const SkRect& rect);