Check delegate_ for null in WebContentsView::GetTopLevelNativeWindow

Bug: 1285467
Change-Id: I2b136d4885c6b479810c41ddd6199c90b27a0bce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3373703


Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Yury Semikhatsky <yurys@chromium.org>
Cr-Commit-Position: refs/heads/main@{#956769}