[mac] Use -[NSApp isActive] for checking if current app is active.

Use short version in RelaunchAppWithHelper() instead of
[[NSRunningApplication currentApplication] isActive]

BUG=650854

Review-Url: https://codereview.chromium.org/2398943002
Cr-Commit-Position: refs/heads/master@{#423716}

chrome/browser/mac/relauncher.mm

@@ -100,7 +100,7 @@ bool RelaunchAppWithHelper(const std::string& helper,
 
   // If this application isn't in the foreground, the relaunched one shouldn't
   // be either.
-  if (![[NSRunningApplication currentApplication] isActive]) {
+  if (![NSApp isActive]) {
     relaunch_args.push_back(kRelauncherBackgroundArg);
   }