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.cpp b/tools/debugger/SkDrawCommand.cpp
index 35e9201..ba6302c 100644
--- a/tools/debugger/SkDrawCommand.cpp
+++ b/tools/debugger/SkDrawCommand.cpp
@@ -428,6 +428,15 @@
     return result;
 }
 
+Json::Value SkDrawCommand::MakeJsonColor4f(const SkColor4f& color) {
+    Json::Value result(Json::arrayValue);
+    result.append(Json::Value(color.fA));
+    result.append(Json::Value(color.fR));
+    result.append(Json::Value(color.fG));
+    result.append(Json::Value(color.fB));
+    return result;
+}
+
 Json::Value SkDrawCommand::MakeJsonPoint(const SkPoint& point) {
     Json::Value result(Json::arrayValue);
     result.append(Json::Value(point.x()));