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This fixes a bug in the uploader where it would post a task with a
callback that had a raw pointer, and if it was during shutdown then the
referenced object could be destructed before the call on said pointer
was invoked.  The simple part of the change is using a WeakPtr in that
location instead of a raw pointer, so the callback will be skipped.
However, the way the code worked was that it would read the pre-existing
feedback report from disk to a string, then it would pass that into the
callback which asynchronously generated a new feedback report in a new
disk location and then it would delete the old feedback report file.
However, if that callback is suppressed (which it would be in the case
the new WeakPtr becomes invalidated) then we would end up deleting that
feedback report from disk without sending it. So this code also changes
things to not recreate the report on disk but instead reuses that
existing file for the feedback report.

Bug: 994846
Test: components_unittests pass and manually verified report upload
Change-Id: Ib64ce1abbfea23e94e45387fa5a296817bf19a30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1766766
Commit-Queue: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691752}