//headless: Use a reduced UA string in HeadlessBrowser

There is no reason for old headless to pass a complete UA string with
accurate OS and CPU architecture information as per the comment in
https://crrev.com/c/6304681.

Bug: 389970327
Change-Id: Id1ade68ab89ac4bcf29ce7bc357c4e1d059136e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6304943
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1425812}

headless/lib/browser/headless_browser_impl.cc

@@ -70,7 +70,7 @@ const base::FilePath::CharType kLocalStateFilename[] =
 }  // namespace
 
 HeadlessBrowser::Options::Options()
-    : user_agent(content::BuildUserAgentFromProduct(
+    : user_agent(content::BuildUnifiedPlatformUserAgentFromProduct(
           HeadlessBrowser::GetProductNameAndVersion())) {}
 
 HeadlessBrowser::Options::Options(Options&& options) = default;