Bug 1568860 - Part 3: Use the contextual WalkerFront in the fonts view. r=ochameau

Depends on D49637

Differential Revision: https://phabricator.services.mozilla.com/D49639

--HG--
extra : moz-landing-system : lando

devtools/client/inspector/fonts/fonts.js

@@ -485,7 +485,7 @@ class FontInspector {
     switch (unit) {
       case "rem":
         // Regardless of CSS property, always use the root document element for "rem".
-        node = await this.inspector.walker.documentElement();
+        node = await this.node.walkerFront.documentElement();
         break;
     }
 
@@ -859,7 +859,7 @@ class FontInspector {
       if (show) {
         const node = isForCurrentElement
           ? this.node
-          : this.inspector.walker.rootNode;
+          : this.node.walkerFront.rootNode;
 
         await this.fontsHighlighter.show(node, {
           CSSFamilyName: font.CSSFamilyName,