[File System] Fix ownership comment in FileSystemOperationRunner.

This comment about ownership was valid for about six weeks when this code
was written six years ago, but then ownership of FileSystemOperationRunner
was changed to no longer always be owned by FileSystemContext. This CL
updates the comment to reflect that.

Using raw pointers here is still mostly safe, as (since recently) all
places that own a FileSystemOperationRunner, also keep FileSystemContext
alive via a scoped_refptr.

Bug: none
Change-Id: Ica49f7062c78ce0f99f7268bebe59750544ae50a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600354
Auto-Submit: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657524}
2 files changed