Add histograms for prerender rel types.

Add histograms for prerender rel types.

Since we have an associated finch experiment, these two histograms
should be good enough for us to catch information on the population
of rel types of prerenders as added and run.

R=tburkard@chromium.org,davidben@chromium.org,isherman@chromium.org
BUG=329963
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=250784

[gavinp, 2013-12-20 19:10:42]

David, Timo: PTAL.

I split out the histograms from the rest to make life easier for our histogram
reviewers, this change is strictly downstream of
https://codereview.chromium.org/113803003/ .

Once the upstream has landed and you folks have gone over this CL, I'll add an
histogram reviewer so we can commit this.

[gavinp, 2013-12-20 19:56:43]

https://codereview.chromium.org/98603015/diff/70002/chrome/browser/prerender/...
File chrome/browser/prerender/prerender_link_manager.cc (right):

https://codereview.chromium.org/98603015/diff/70002/chrome/browser/prerender/...
chrome/browser/prerender/prerender_link_manager.cc:62: enum RelTypeHistogramEnum
{
Our coding standard doesn't define an order for translation units, only for
classes. The restriction on ordering is within classes, see
http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml?showone=Declar...
. 

So I moved the enum down here, closer to the histogram reporting functions, for
easier readability. What do you think?

[davidben, 2014-01-02 19:30:26]

LGTM. The bitmask enum feels pretty goofy, but I guess they have to be listed
out somewhere for the histograms stuff. I'll leave it to the histograms reviewer
for whether that's the usual pattern.

Nit: s/prerendres/prerenders/ in the description.

[gavinp, 2014-02-07 20:37:38]

+isherman

isherman: PTAL for histograms.

[Ilya Sherman, 2014-02-07 21:32:58]

histograms lgtm

https://codereview.chromium.org/98603015/diff/540001/tools/metrics/histograms...
File tools/metrics/histograms/histograms.xml (right):

https://codereview.chromium.org/98603015/diff/540001/tools/metrics/histograms...
tools/metrics/histograms/histograms.xml:30017: +  <int value="3"
label="PRERENDER_REL_TYPE_PRERENDER_AND_NEXT"/>
Optional nit: These labels show as human-readable text, so you might want to
format these in a more human-friendly way.

[gavinp, 2014-02-12 17:08:26]

The CQ bit was checked by gavinp@chromium.org

[commit-bot: I haz the power, 2014-02-12 17:10:27]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/gavinp@chromium.org/98603015/540001

[commit-bot: I haz the power, 2014-02-12 20:10:00]

Change committed as 250784