Bug 1460456 part 1 - Change the semantics of 'auto' color value. r=heycam

MozReview-Commit-ID: CRkhO35MoB8

--HG--
extra : rebase_source : ad4e88365c428fe081ec55897ec8260e6102595c

layout/style/StyleComplexColor.h

@@ -23,15 +23,19 @@ class ComputedStyle;
  * Conceptually, the formula is "color * (1 - p) + currentcolor * p"
  * where p is mForegroundRatio. See mozilla::LinearBlendColors for
  * the actual algorithm.
+ *
+ * It can also represent an "auto" value, which is valid for some
+ * properties. See comment of mIsAuto.
  */
 struct StyleComplexColor
 {
   nscolor mColor;
   uint8_t mForegroundRatio;
   // Whether the complex color represents a computed-value time auto
-  // value. This is only a flag indicating that this value should not
-  // be interpolatable with other colors, while other fields still
-  // represents the actual used color of this value.
+  // value. This is a flag indicating that this value should not be
+  // interpolatable with other colors. When this flag is set, other
+  // fields represent a currentcolor. Properties can decide whether
+  // that should be used.
   bool mIsAuto;
 
   static StyleComplexColor FromColor(nscolor aColor) {