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The problem is that we were generating zero hashes when there
were no input bytes, when we should have generated one.

To make this fix a little easier to test, I moved the code for
generating hashes of content into a utility funciton in
computed_hashes.{h,cc}. Also, in order to fix things for any
profiles that might have incorrect computed_hashes.json files, I
changed the format of it to add the notion of a version. When the
format or version isn't recognized, the code will return an error
from the Init function, and then the code that uses it elsewhere
in the content verification code will automatically try to
recreate it.

BUG=399251

Review URL: https://codereview.chromium.org/436563004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287740 0039d316-1c4b-4281-b951-d872f2087c98