iwa: Let Web Bundle Reader to close the parser

Let the reader to close the parser. This is one of the steps to
closing the Signed Web Bundle file and so that we can remove it
afterwards. In the upcoming CLs the reader will also close:
- the file that is being validated;
- the DataPipeProducer that is used for reading response body.

Bug: b/287594073
Test: added unittests

Change-Id: I52e39cceaaaa5b19bdb66940dd0e7210b59faa80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4856865
Reviewed-by: Christian Flach <cmfcmf@chromium.org>
Auto-Submit: Oleksandr Peletskyi <peletskyi@google.com>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Oleksandr Peletskyi <peletskyi@google.com>
Cr-Commit-Position: refs/heads/main@{#1197873}