Provide the OS username on more platforms

For Chrome OS we just get it from the active user. For other platforms
we do the same but only as a best effort since we are not testing it,
so after a NOTREACHED(), and replacing the current best-effort return
of an empty string.

Bug: chromium:1012048
Test: browser_tests
Change-Id: I8af942cfb01f9be6c0061b1271a395fe3768f1b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1859676Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Yves Arrouye <drcrash@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706250}

components/policy/core/common/BUILD.gn

@@ -156,6 +156,7 @@ jumbo_source_set("internal") {
     "//components/policy:generated",
     "//components/policy/proto",
   ]
+
   deps = [
     "//base:i18n",
     "//base/third_party/dynamic_annotations",
@@ -163,6 +164,7 @@ jumbo_source_set("internal") {
     "//components/prefs",
     "//components/signin/public/identity_manager",
     "//components/strings",
+    "//components/user_manager",
     "//components/version_info",
     "//extensions/buildflags",
     "//google_apis",

components/policy/core/common/cloud/DEPS

 include_rules = [
   "+components/signin/public",
+  "+components/user_manager",
   "+services/identity/public/cpp",
 ]

components/policy/core/common/cloud/cloud_policy_util.cc

@@ -39,6 +39,11 @@
 #include "base/system/sys_info.h"
 #include "components/version_info/version_info.h"
 
+#if defined(OS_CHROMEOS)
+#include "components/user_manager/user.h"
+#include "components/user_manager/user_manager.h"
+#endif
+
 #if defined(OS_WIN)
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
@@ -153,6 +158,13 @@ std::string GetOSUsername() {
   }
 
   return base::WideToUTF8(username);
+#elif defined(OS_CHROMEOS)
+  if (!user_manager::UserManager::IsInitialized())
+    return std::string();
+  auto* user = user_manager::UserManager::Get()->GetPrimaryUser();
+  if (!user)
+    return std::string();
+  return user->GetAccountName(/*use_display_email=*/false);
 #else
   NOTREACHED();
   return std::string();