Remove Net.URLRequest_SetReferrer_IsEmptyOrValid histogram.

Remove Net.URLRequest_SetReferrer_IsEmptyOrValid histogram.

According to internal stats, this was the top 5 histogram
that's taking up space in UMA and it was never documented
in histograms.xml, which means no one has ever looked at
its data. Delete it so that it stop taking resources without
providing value.

Document it in histograms.xml so that previous data can
still be viewed on the dashboard, if needed.

BUG=387734
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=279639

[cbentzel, 2014-06-24 13:11:22]

+mef: Do you remember why this was put in?

LGTM

[mef, 2014-06-24 14:05:17]

On 2014/06/24 13:11:22, cbentzel wrote:
> +mef: Do you remember why this was put in?
> 
> LGTM

According to this: https://chromiumcodereview.appspot.com/12569007 the idea was
to
see how often are invalid urls seen as referrers, so potentially change
referrer_ type from std::string to GURL.

I think that was my starter bug, so I doubt that it was my idea, but I can't
find hard evidence to contrary. :-)

Removal lgtm, and sorry for taking all that space.

[Alexei Svitkine (slow), 2014-06-25 02:03:17]

The CQ bit was checked by asvitkine@chromium.org

[commit-bot: I haz the power, 2014-06-25 02:06:37]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/asvitkine@chromium.org/331343003/1

[commit-bot: I haz the power, 2014-06-25 06:43:13]

Change committed as 279639