Fix crash in BubbleDialogDelegate with null client view

Check if GetBubbleFrameView() is null before calling SizeToContents
in BubbleDialogDelegate::OnAnchorBoundsChanged. This should fix a
crash when a nested message loop causes OnWindowPosChanged to
be called before the widget has initialized its non_client_view_.

Bug: 395293617
Change-Id: I457fd1362d194ee8ed2e0a6933e8ee03690bd13a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6334852
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1429454}