Change the libtxt default text color to white in order to match Rende…

…rStyle::initialColor (flutter#4074)

third_party/txt/src/txt/text_style.h

@@ -29,7 +29,7 @@ namespace txt {
 
 class TextStyle {
  public:
-  SkColor color = SK_ColorBLACK;
+  SkColor color = SK_ColorWHITE;
   TextDecoration decoration = TextDecoration::kNone;
   // Does not make sense to draw a transparent object, so we use it as a default
   // value to indicate no decoration color was set.