[StartRefactor] Fix testSingleAsHomepage_Landscape_TabSize.

This test failed when enabling the refactor flag. In this CL, we fix it,
and make it tested with/without Start surface refactor is enabled.

Bug: 1347089
Change-Id: I6bfd061325116ebf6f437fd61f8ddf0119c6e89e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4569283


Commit-Queue: Xi Han <hanxi@chromium.org>
Reviewed-by: Xinyi Ji <xinyiji@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149410}