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Now that FileStreamReader and FileStreamWriter work with
PluginPrivateFileSystem, enable incognito mode for
PluginPrivateFileSystem.

Also updates FileStreamReader to work with the PluginPrivateFileSystem
in incognito mode, as the ObfuscatedFileUtilMemoryDelegate is
different.

This adds 2 more browser tests that verify that encrypted media
can read and write files successfully in incognito mode (as it
uses the PluginPrivateFileSystem). Verified using inotifywait that
the test does not write anything to disk.

Bug: 958294,902021
Test: new browser_tests pass
Change-Id: I01914cdd866ff96867e7bebfeaea6ef827a5a9fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1718985Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682094}