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

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247790 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247790