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Servers set the download filename in a URL syntax (either through the
Content-Disposition header, or in the filename of the requested URL
itself). Previously, characters would remain percent-encoded when saved
to disk, if they are deemed unsuitable for display. This criteria makes
no sense for setting a filename.

Cases included:
- Control characters.
- Path separators.
- Special URL characters such as '%', '+', '&' and '#'.
- Spoofing characters, such as invisible characters.

Now, all characters are unescaped. Characters that are illegal in
filenames (such as control characters, path separators, colons, etc) are
converted into underscores. Other characters are left bare.

Fixed and added tests.

Bug: 849794
Change-Id: I797ee3d6aa8b803d9a7227e821fd8b4d55d0c58d
Reviewed-on: https://chromium-review.googlesource.com/1209003Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Matt Giuca <mgiuca@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589386}