Don't use NotificationStateLock for GetMainFrame (see issue #2421)

This causes a race related to |notification_state_lock_| assignment when
GetMainFrame is called from multiple threads. GetMainFrame doesn't
trigger any notifications so it shouldn't need that lock. Instead, only
use NotificationStateLock on the UI thread.

libcef/browser/browser_info.h

@@ -204,7 +204,7 @@ class CefBrowserInfo : public base::RefCountedThreadSafe<CefBrowserInfo> {
   // Used instead of |base::AutoLock(lock_)| in situations that might generate
   // CefFrameHandler notifications. Any notifications passed to
   // MaybeExecuteFrameNotification() will be queued until the lock is released,
-  // and then executed in order.
+  // and then executed in order. Only accessed on the UI thread.
   class NotificationStateLock final {
    public:
     explicit NotificationStateLock(CefBrowserInfo* browser_info);