Add EnableFilesystemInIncognito feature and flag.

EnableFilesystemInIncognito feature and flag are added to control
activation of in-memory filesystem api for incognito mode.

Bug: 93417
Change-Id: I7002ff23afd0d1ceacf29be3b01ce6f2cd09410c
Reviewed-on: https://chromium-review.googlesource.com/c/1472290
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632512}
8 files changed