[cr145] Contextrual search service factory gn target modev.

Chromium change:

https://source.chromium.org/chromium/chromium/src/+/9a3f20f017c15681e9e3395a079fd57aa1170484

commit 9a3f20f017c15681e9e3395a079fd57aa1170484
Author: Juan Mojica <juanmojica@google.com>
Date:   Mon Nov 24 02:22:10 2025 -0800

    [contextual-search] Move ContextualSearchServiceFactory to contextual_search target.

    The existing "web_contents_helper" target is renamed to
    "contextual_search" and now includes both the service factory and the
    web contents helper. Dependencies on the old target are updated
    accordingly

browser/ui/BUILD.gn

@@ -397,6 +397,7 @@ source_set("ui") {
       "//chrome/app:generated_resources",
       "//chrome/browser/app_mode",
       "//chrome/browser/content_settings:content_settings_factory",
+      "//chrome/browser/contextual_search",
       "//chrome/browser/favicon",
       "//chrome/browser/importer",
       "//chrome/browser/profiles",