merge_pr_49008

Add a test updating scroll position mid-smooth-scroll

This is a minimized test of https://crbug.com/376097633 on
boostmobile.com.

Bug: 376097633
Change-Id: I014d2a1b276f8047272f2a1bf587233844574951
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6001172
Commit-Queue: Philip Rogers [email protected]
Commit-Queue: Xianzhu Wang [email protected]
Reviewed-by: Xianzhu Wang [email protected]
Auto-Submit: Philip Rogers [email protected]
Cr-Commit-Position: refs/heads/main@{#1379136}