A bunch of remaining parts of extension content verification

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

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270694 0039d316-1c4b-4281-b951-d872f2087c98