Add MemoryFileStream[Reader/Writer].

MemoryFileStreamReader and MemoryFileStreamWriter classes are added to
read/write from memory files. When a new file stream reader/writer is
asked to be created, file system backend is queried if the operating is
in incognito mode or regular mode and if it's incognito, the
MemoryFileStream* ones are created instead of LocalFileStream* ones.

The change is part of the following design doc:

Bug: 93417
Change-Id: Ia55f085584a5ac699fad7d368936e467c60519dd
Commit-Queue: Ramin Halavati <>
Reviewed-by: Marijn Kruisselbrink <>
Cr-Commit-Position: refs/heads/master@{#632556}
18 files changed