Allow platforms to wait before the website screenshot is taken.

In the Finch Smoke tests for the Android tests we experience
a lot of false negatives when it comes to this unit test.
The reason is different color of the scroll bar. Because it
slowly fades, its color on the screenshot depends on the timing.
We add sleep clause to mitigate this effect.


Change-Id: I6f2dd529546ff5078e8c03a89612a6b35554b0e6
Bug: 413580167
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6487806
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Cr-Commit-Position: refs/heads/main@{#1452716}