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There are two problems with the CheckClientDownloadRequest that this CL
takes steps towards resolving. First, the control flow of the
CheckClientDownloadRequest is very difficult to follow due to a large
number of private helper methods. Secondly, it uses several techniques
to prevent use-after-free (ref-counting, weak pointers, and cancellable
tasks). This CL encapsulates the logic that was in
CheckClientDownloadRequest that was related to analyzing the content
of the file into a singly-owned class, so that those two concerns can
be more easily addressed.

TODO: The FileAnalyzer still needs to be tested for RAR files.

Bug: 889986
Change-Id: I81c02bd3491c116a949dc959f2f02208511209bd
Reviewed-on: https://chromium-review.googlesource.com/c/1272275
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600785}