Fix crash when enterprise.reportingPrivate API can't retrieve password

RetrieveDeviceSecret doesn't quit properly on Mac when password can't
be retrieved. It invokes the |callback| twice and causes CHECK failure.

Verify:
1) Install extension that has reportingPrivate permssion.
2) Open Console
3) Run following command:
chrome.enterpriase
  .reportingPrivate.getPersistentSecret((i)=>{console.log(i);});

TEST: Manually
Bug: 1068329
Change-Id: Ic12665c81a5794268e02d1208a20599aef48f500
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153893Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760033}

chrome/browser/extensions/api/enterprise_reporting_private/chrome_desktop_report_request_helper.cc

@@ -531,8 +531,10 @@ void RetrieveDeviceSecret(
   }
 #elif defined(OS_MACOSX)
   secret = ReadEncryptedSecret();
-  if (secret.empty())
+  if (secret.empty()) {
     std::move(callback).Run(secret, false);
+    return;
+  }
 
 #endif
   std::move(callback).Run(secret, true);