Bug 1532850 - Check the return value of LookAndFeel::GetInt() in case of the function failure. r=emilio

Actually the function will fail on Android since on xpcshell tests we don't
initialize AndroidBridge so that we can't query the corresponding system
settings.

Differential Revision: https://phabricator.services.mozilla.com/D22270

--HG--
extra : moz-landing-system : lando

layout/style/nsMediaFeatures.cpp

@@ -462,8 +462,9 @@ void nsMediaFeatures::InitSystemMetrics() {
         (nsStaticAtom*)nsGkAtoms::_moz_gtk_csd_reversed_placement);
   }
 
-  metricResult = LookAndFeel::GetInt(LookAndFeel::eIntID_SystemUsesDarkTheme);
-  if (metricResult) {
+  rv = LookAndFeel::GetInt(LookAndFeel::eIntID_SystemUsesDarkTheme,
+                           &metricResult);
+  if (NS_SUCCEEDED(rv) && metricResult) {
     sSystemMetrics->AppendElement(
         (nsStaticAtom*)nsGkAtoms::_moz_system_dark_theme);
   }