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It seems that corruption examples in crbug.com/699420 can cause extension
resource files to go missing or become unreadable.
Fix a bug where we never sent any notification to ContentVerifyJob,
in particular: we never called ContentVerifyJob::DoneReading().

Override URLRequestFileJob::OnOpenComplete in URLRequestExtensionJob
to catch these cases.

Also make ContentHashReader not skip legitimate deleted files: the
files that are listed in verified_contents.json. It will still skip
non-existent resources, e.g. xhrs requesting non-existent file (see
crbug.com/404802 for example).

BUG=703888
Test=Install an extension (not app) in chromeos. Chromeos because you want
content verification enforcement to kick in. Other way could be to use
--extension-content-verification=enforce_strict flag in other platform.
Either:
1) Delete background script file. Content verifcation would kick in when
you try to load/run the extension.
2) Same as above^^^, instead of deleting the file, remove the file
permission. In linux you can just do "chmod -r background.js". Expect
content verification failure.
In both cases extension should be reinstalled in the end.

Review-Url: https://codereview.chromium.org/2771953003
Cr-Commit-Position: refs/heads/master@{#460607}