Highlight API: Implement devtools support

Bug: 1195196
Change-Id: I8ecc4b63e1502e2555c65ca324450ab1ffe16cc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2998242
Reviewed-by: Dan Clark <daniec@microsoft.com>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Fernando Fiori <ffiori@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#898375}

css/css-pseudo/highlight-pseudos-computed.html

@@ -8,14 +8,19 @@
   #target::selection,
   #target::target-text,
   #target::spelling-error,
-  #target::grammar-error {
+  #target::grammar-error,
+  #target::highlight(foo) {
     background-color: green;
     color: lime;
   }
+  #target::highlight(bar) {
+    background-color: blue;
+    color: yellow;
+  }
 </style>
 <div id="target"></div>
 <script>
-  for (const pseudo of ["::selection", "::target-text", "::spelling-error", "::grammar-error"]) {
+  for (const pseudo of ["::selection", "::target-text", "::spelling-error", "::grammar-error", "::highlight(foo)"]) {
     test(() => {
       let style = getComputedStyle(target, pseudo);
       assert_equals(style.backgroundColor, "rgb(0, 128, 0)", "Background color is green.");
@@ -31,4 +36,11 @@
       }, `getComputedStyle() for ${illFormedPseudo} should be element's default`);
     }
   }
+
+  const pseudo = "::highlight(bar)";
+  test(() => {
+    let style = getComputedStyle(target, pseudo);
+    assert_equals(style.backgroundColor, "rgb(0, 0, 255)", "Background color is blue.");
+    assert_equals(style.color, "rgb(255, 255, 0)", "Color is yellow.");
+  }, `Different getComputedStyle() for ${pseudo} and same element`);
 </script>