Attempt to remove flakiness of SensitiveContentTest

SensitiveContentTest#testSwipingBetweenTabsIsSensitive was flaky. The
failure always happened among the last swipes. Reducing the number of
swipes might remove the failure.

The exact reason for failure is unclear, as it is not reproduceable
locally, but I suspect swiping left and right many times might result in
some swipes not being properly ended, and another swipe cannot start.

Bug: 378742136
Change-Id: Ie1d1a7a088e48e7432f0e8c8560fdabddc5d7c08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6038521
Commit-Queue: Theo Cristea <theocristea@google.com>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1390225}