cros: Fix unit_tests crashes in GetBrand() on official builders

unit_tests do not have prefs, but branded builds on Chrome OS attempt to read
the brand code out of a null g_browser_process->local_state().

Instead, we now return an empty brand code in tests.

This reduces the number of crashing unit_tests on the official builders from
~300 to ~30.

BUG=430266

Review URL: https://codereview.chromium.org/703623005

Cr-Commit-Position: refs/heads/master@{#302733}

chrome/browser/google/google_brand_chromeos.cc

@@ -59,7 +59,9 @@ std::string GetBrand() {
          content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
   if (g_brand_empty)
     return std::string();
-  DCHECK(g_browser_process->local_state());
+  // Unit tests do not have prefs.
+  if (!g_browser_process->local_state())
+    return std::string();
   return g_browser_process->local_state()->GetString(prefs::kRLZBrand);
 }