Do not display crOS Account Manager for background navigations

Navigating to Gaia URLs like /AddSession opens the Chrome OS Account
Manager (for adding a session) but we want to ensure that this does not
happen for background navigations.

Please check the attached bug for context.

Bug: 945218
Change-Id: I0cafbe62b0215a9bcd9e8190031c9baab9c96466
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1561139Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649823}

chrome/browser/signin/chrome_signin_helper.cc

@@ -210,6 +210,12 @@ void ProcessMirrorHeaderUIThread(
     //
     // - Going Incognito (already handled in above switch-case).
     // - Displaying the Account Manager for managing accounts.
+
+    // Do not display Account Manager if the navigation happened in the
+    // "background".
+    if (!chrome::FindBrowserWithWebContents(web_contents))
+      return;
+
     chrome::SettingsWindowManager::GetInstance()->ShowChromePageForProfile(
         profile, GURL("chrome://settings/accountManager"));
     return;