Revert 270694 "A bunch of remaining parts of extension content v..."

Revert 270694 "A bunch of remaining parts of extension content v..."

> A bunch of remaining parts of extension content verification
>
> -The real guts of content_hash_fetcher.cc, which fetches the
>  verified_contents.json file from the webstore if needed and also runs
>  tasks to compute and cache the block-level hashes of all files in an
>  extension.
>
> -The real guts of content_hash_reader.cc, which uses the work done by
>  the content_hash_fetcher during validation of extension file content as
>  it's read off of disk at time of use.
>
> -Code to avoid verifying transcoded files (images used in browser
>  process, and message catalogs).
>
> -Don't allow downgrade of mode via kForceFieldTrials command line switch
>
> -Various bits of plumbing to support all of the above
>
> BUG=369895
> R=rockot@chromium.org
>
> Review URL: https://codereview.chromium.org/289533003

TBR=asargent@chromium.org

Review URL: https://codereview.chromium.org/284223002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270734 0039d316-1c4b-4281-b951-d872f2087c98