[SxS] Center tab modal dialogs to corresponding ContentsContainerView

Tab modal dialogs should be centered in the contents container view
for the tab that triggered that dialog; this is important for split
view where multiple tabs can be shown at the same time.

Fixed: 422176169
Change-Id: I0f6b2241cc912a9ab69c30ae5410a8f742e3fe0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6886576
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Reviewed-by: Darryl James <dljames@chromium.org>
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Commit-Queue: Steven Luong <stluong@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1508095}