Move incognito file system usage cache to memory.

Incognito filesystem's usage cache is moved to memory and a local
file is not generated for that anymore.

Bug: 93417
Change-Id: I86186838a21531f52baffd803ba49db89f0d35d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1569937
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Auto-Submit: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653929}
7 files changed