Chrome OS: Fix crash when removing mangification handler

Bug: 1138781
Change-Id: Ibf8c9594777db2b6f0fd813ac9320962acf62734
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2535643Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Josiah Krutz <josiahk@google.com>
Cr-Commit-Position: refs/heads/master@{#827134}

chrome/browser/chromeos/chrome_browser_main_chromeos.cc

@@ -1134,6 +1134,7 @@ void ChromeBrowserMainPartsChromeos::PostMainMessageLoopRun() {
 
   assistant_state_client_.reset();
 
+  if (pre_profile_init_called_)
     ash::Shell::Get()->RemovePreTargetHandler(MagnificationManager::Get());
 
   // Unregister CrosSettings observers before CrosSettings is destroyed.