[SessionStorage] Always delete Session Storage database on Android

Android doesn't have session restore code run, and this code is what
cleaned up old session storage namespaces from previous browser runs.

So on Android, the session storage context will delete the database
on startup.

R=mek@chromium.org

Bug: 860030
Change-Id: Ia7c349811e4970d05a071a0de9ad50117d771855
Reviewed-on: https://chromium-review.googlesource.com/1125260
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574673}
4 files changed