[cr143] Use the default GetUserAgentMetadata variant

There used to be another overload for this function, which is now gone.
This code corrects the caller to use the only remaining variant with the
default value.

Chromium changes:
https://chromium.googlesource.com/chromium/src/+/e18585881702f3b142d71c87f068707fd1104a4e

commit e18585881702f3b142d71c87f068707fd1104a4e
Author: Henrique Ferreiro <hferreiro@igalia.com>
Date:   Tue Sep 30 03:33:31 2025 -0700

    Remove unused embedder_support::GetUserAgentMetadata variant

    https://crrev.com/c/6205906 removed the only usage of the pref service
    so we don't need to pass it as an argument to GetUserAgentMetadata.

    Bug: 1261908
    Change-Id: I4cb4dedb4ae43bb95f70b22baaf6959e4c93cba4
    Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6964493
    Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
    Reviewed-by: Eric Seckler <eseckler@chromium.org>
    Reviewed-by: Nate Fischer <ntfschr@chromium.org>
    Reviewed-by: Victor Tan <victortan@chromium.org>
    Cr-Commit-Position: refs/heads/main@{#1522762}

components/embedder_support/user_agent_utils_unittest.cc

@@ -47,7 +47,7 @@ TEST(UserAgentUtilsTest, UserAgentFromCommandLine) {
                                                           kCmdUserAgentValue);
   ASSERT_TRUE(
       base::FeatureList::IsEnabled(blink::features::kUACHOverrideBlank));
-  const auto brave_metadata = GetUserAgentMetadata(nullptr);
+  const auto brave_metadata = GetUserAgentMetadata();
   const blink::UserAgentMetadata empty_metadata;
 
   EXPECT_EQ(GetUserAgent(), kCmdUserAgentValue);