Prepend starboard OS name in user-agent string

[santoshmahto7]

The current OS name in user-agent string is fetched using
chromium code which just return "Linux". This misses the real
OS name for starboard platform and result in YTS test failure.
So OS name in user-agent string should include the starboard platform
OS name.

Bug: 475670748

[github-actions]

🤖 Gemini Suggested Commit Message

---

starboard: Prepend OS name to user-agent string

The user-agent string previously used base::SysInfo::OperatingSystemName(),
which reports "Linux" for Starboard platforms. This generic identification
was insufficient and led to YTS test failures that rely on specific OS
identification. Prepend the Starboard platform's friendly name to the OS
string to provide accurate platform information.

Bug: 475670748

---

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[fayev-g]

lgtm, as tvOS will be addressed with b/483228604

[jellefoks]

This LGTM

[niranjanyardi]

causes build error:

./../third_party/musl/src/starboard/include -isystem../../third_party/musl/include -isystem../../third_party/musl/arch/x86_64 -isystem../../third_party/musl/arch/generic -femulated-tls -fvisibility-inlines-hidden -c ../../base/system/sys_info_starboard_unittest.cc -o obj/base/base_unittests/sys_info_starboard_unittest.o
../../base/system/sys_info_starboard_unittest.cc:25:55: error: expected value in expression
#if BUILDFLAG(IS_ANDROID) || BUILDFLAG(IS_IOS_TVOS) ||