Fix several problems with the content verification code

-While we're doing the initial scan of files to build up
 computed_hashes.json, if we notice a mismatch, report that right
 away. This also fixes problems where if you modify/delete
 computed_hashes.json, enforcement would never work.

-Avoid an infinite loop in ContentVerifyJob by making sure we stop
 checking newly read bytes after we've already reported a failure
 (we were stuck failing to advance to the next block after reporting
  a failure)

BUG=375992,375953

Review URL: https://codereview.chromium.org/329303007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278071 0039d316-1c4b-4281-b951-d872f2087c98
9 files changed