Revert 247790 "Change default mode of extension install verifica..."

> Change default mode of extension install verification
> 
> Right now the default mode is to make requests against the server unless
> the experiment group says not to, and that may be contributing to some
> unexpected load we're seeing. So this CL changes it to not do that.
> 
> It also adds some histograms to help measure the success rate and
> sources of requests to the server.
> 
> BUG=335379
> R=finnur@chromium.org
> 
> Review URL: https://codereview.chromium.org/149353002

TBR=asargent@chromium.org

Review URL: https://codereview.chromium.org/147983011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247797 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247797