Change default mode of extension install verification

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.

[This is a resubmit of https://codereview.chromium.org/149353002 with a
compile error fixed]

BUG=335379
TBR=finnur@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=247954

[asargent_no_longer_on_chrome, 2014-01-30 06:22:36]

Trying this again, hopefully now with 100% less compile failures.

[commit-bot: I haz the power, 2014-01-30 06:26:09]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asargent@chromium.org/150093002/10001

[commit-bot: I haz the power, 2014-01-30 07:58:15]

Retried try job too often on android_dbg for step(s) slave_steps
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=android_db...

[Finnur, 2014-01-30 10:02:21]

LGTM

[commit-bot: I haz the power, 2014-01-30 10:03:44]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asargent@chromium.org/150093002/10001

[commit-bot: I haz the power, 2014-01-30 13:55:47]

Change committed as 247954