[partition-as-single] Add FilesSinglePartitionFormat feature flag.

Adds util method and integration test load time data entry to check
feature in the Foreground page as well.

Bug: 491043
Change-Id: Ia10eb456a982d2f8d8a4ce09d3c80d0b82f5e2fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2398598


Reviewed-by: Sergei Datsenko <dats@chromium.org>
Reviewed-by: Austin Tankiang <austinct@chromium.org>
Commit-Queue: Omid Tourzan <oto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805660}