[fuchsia][webengine] Expose fuchsia.web.Debug from web_instance

Otherwise, WebInstanceHost consumers that attempt to attach to it will
fail to do so.

Bug: 1280703
Change-Id: I1849911c3c794904ca3c434b66ec531b2ced9aed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4271114
Commit-Queue: Wez <wez@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1107435}

fuchsia_web/webengine/web_instance.cml

@@ -17,6 +17,7 @@
     {
       protocol: [
         "fuchsia.web.Context",
+        "fuchsia.web.Debug",
         "fuchsia.web.FrameHost",
       ]
     }
@@ -25,6 +26,7 @@
     {
       protocol: [
         "fuchsia.web.Context",
+        "fuchsia.web.Debug",
         "fuchsia.web.FrameHost",
       ],
       from: "self",