Force outline color at used value time

Forced colors should now happen at used value time. This change adds
an internal property to store the forced colors value of
outline-color. This internal value is then applied to the visited
/unvisited outline-color at used value time when appropriate.

The other forced color properties will be moved in follow up changes.

Bug: 970285
Change-Id: I100438fbf9d5230f905a7d16067d387c65876448
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518280
Commit-Queue: Alison Maher <almaher@microsoft.com>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829961}

forced-colors-mode/forced-colors-mode-40.html

@@ -11,6 +11,7 @@
     background-color: green;
     border-color: green;
     fill: green;
+    outline-color: green;
     stroke: green;
   }
 </style>
@@ -26,6 +27,7 @@
     "border-right-color",
     "border-top-color",
     "fill",
+    "outline-color",
     "stroke"
   ];
   for (let property of properties_to_test) {

forced-colors-mode/forced-colors-mode-41.html

@@ -12,6 +12,7 @@
   div {
     background-color: green;
     border-color: green;
+    outline-color: green;
   }
 </style>
 <div id="div"></div>
@@ -23,6 +24,7 @@
     "border-left-color",
     "border-right-color",
     "border-top-color",
+    "outline-color"
   ];
   for (let property of properties_to_test) {
     test(function() {