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AbstractFile.overwriteOriginal is implemented in the guest context to
send IPC to the privileged host to achieve the file write. To ensure
the correct file is written, a token is passed on IPC load requests.

Only the file that the privileged context determines is "current" may
be written to by the unprivileged guest.

Testing
 - Remove receiver_api.js and depend on receiver.js directly for
   type checking.
 - Mock out the native file system on the host side.
 - Add a test-only IPC to trigger AbstractFile.overwriteOriginal().

Bug: b/146580738
Change-Id: Ie68213e3e1412a2d1fd63d8039d71858f5b4c740
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090992
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: Bugs Nash <bugsnash@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748428}