Store keyword for unresolved StyleColor.

We currently store a bool saying if the StyleColor represents
currentcolor or not since currentcolor computes to itself and will
inherit as currentcolor. If [1] is resolved, we need to do the same for
system colors. Store a CSSValueID in preparation for that.

This CL should not have any behavioral changes.

[1] https://github.com/w3c/csswg-drafts/issues/3847

Bug: 939811
Change-Id: I5908a09b311c549c9d194bdf4180e24ba7baae5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1771413Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690897}

third_party/blink/renderer/core/css/style_color.h

@@ -42,29 +42,34 @@ class StyleColor {
   DISALLOW_NEW();
 
  public:
-  StyleColor() : current_color_(true) {}
-  StyleColor(Color color) : color_(color), current_color_(false) {}
+  StyleColor() : color_keyword_(CSSValueID::kCurrentcolor) {}
+  StyleColor(Color color)
+      : color_(color), color_keyword_(CSSValueID::kInvalid) {}
   static StyleColor CurrentColor() { return StyleColor(); }
 
-  bool IsCurrentColor() const { return current_color_; }
+  bool IsCurrentColor() const {
+    return color_keyword_ == CSSValueID::kCurrentcolor;
+  }
   Color GetColor() const {
     DCHECK(!IsCurrentColor());
     return color_;
   }
 
   Color Resolve(Color current_color) const {
-    return current_color_ ? current_color : color_;
+    return IsCurrentColor() ? current_color : color_;
   }
 
-  bool HasAlpha() const { return !current_color_ && color_.HasAlpha(); }
+  bool HasAlpha() const { return !IsCurrentColor() && color_.HasAlpha(); }
 
   static Color ColorFromKeyword(CSSValueID, WebColorScheme color_scheme);
   static bool IsColorKeyword(CSSValueID);
   static bool IsSystemColor(CSSValueID);
 
  private:
+  explicit StyleColor(CSSValueID keyword) : color_keyword_(keyword) {}
+
   Color color_;
-  bool current_color_;
+  CSSValueID color_keyword_;
 };
 
 inline bool operator==(const StyleColor& a, const StyleColor& b) {