ChromeOS: Allow partial (uptime only) Uptime.Logout UMA metrics.

BUG=407388
TEST=manual

Review URL: https://codereview.chromium.org/518843002

Cr-Commit-Position: refs/heads/master@{#292866}

chrome/browser/chromeos/boot_times_loader.cc

@@ -145,7 +145,7 @@ BootTimesLoader::Stats BootTimesLoader::Stats::GetCurrentStats() {
 }
 
 std::string BootTimesLoader::Stats::SerializeToString() const {
-  if (uptime_.empty() || disk_.empty())
+  if (uptime_.empty() && disk_.empty())
     return std::string();
   base::DictionaryValue dictionary;
   dictionary.SetString(kUptime, uptime_);