fonts: Store web fonts in the per-Layout FontContext (#32303)

This moves mangement of web fonts to the per-Layout `FontContext`,
preventing web fonts from being available in different Documents.

Fixes #12920.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Mukilan Thiyagarajan <mukilan@igalia.com>

tests/wpt/meta/MANIFEST.json

@@ -174727,6 +174727,19 @@
        {}
       ]
      ],
+     "downloadable-font-scoped-to-document.html": [
+      "2dbc350069cd9c61925967655f83217b800fc9eb",
+      [
+       null,
+       [
+        [
+         "/css/css-fonts/downloadable-font-scoped-to-document-ref.html",
+         "=="
+        ]
+       ],
+       {}
+      ]
+     ],
      "first-available-font-001.html": [
       "5eb88f7bf6713b80e0adb4728681e62c3f2dc2bc",
       [
@@ -396276,6 +396289,10 @@
       "28a92a86dcaf6bc9c45bb75fce4869bc0ae21c37",
       []
      ],
+     "downloadable-font-scoped-to-document-ref.html": [
+      "4d7da060cbf1a161aa1366f0bcb00b94e09502ad",
+      []
+     ],
      "first-available-font-001-ref.html": [
       "0acbd338e0ce9f558d2eaa2e48ad4be0524fb0ae",
       []
@@ -403399,6 +403416,18 @@
        "6ed4aa506e95a35a065318f597547653bda52eb5",
        []
       ],
+      "iframe-missing-font-face-rule.html": [
+       "da97b781e8072923138e0160320e76d3013c3e53",
+       []
+      ],
+      "iframe-using-ahem-as-web-font.html": [
+       "b21066df8f57a8b11432a1168a62e7a4fbbe07b1",
+       []
+      ],
+      "iframe-without-web-font.html": [
+       "85e7fef282889894016088e082b623b92a436784",
+       []
+      ],
       "js": {
        "font-variant-features.js": [
         "4b56fee193956710b847ba79c5f9c3a5a7d15a33",