Fix prerender histograms for multiple prerender case.

chrome/browser/prerender/prerender_origin.h

Index: chrome/browser/prerender/prerender_origin.h
diff --git a/chrome/browser/prerender/prerender_origin.h b/chrome/browser/prerender/prerender_origin.h
index 0c79daa9ebbb63d79eb630f1c7c53d43ae91d88c..37c534b9ef604631a213080d5795be10efec4a1f 100644
--- a/chrome/browser/prerender/prerender_origin.h
+++ b/chrome/browser/prerender/prerender_origin.h
@@ -16,7 +16,9 @@ enum Origin {
   // Obsolete: ORIGIN_OMNIBOX_CONSERVATIVE = 3,
   // Obsolete: ORIGIN_OMNIBOX_EXACT = 4,
   ORIGIN_OMNIBOX = 5,
-  ORIGIN_MAX
+  ORIGIN_LOCAL_PREDICTOR = 6,

[mmenke, 2012/10/08 18:20:07]

This origin is currently not used in the same histogram as any other origin. 
It's fed directly to 3 histogram functions not used with anything else, and has
no line in PREFIXED_HISTOGRAM_INTERNAL.

It looks like it works fine, this just seems a little odd, and fragile.  If the
predictor is ever enabled, I could see it being switched to this code without
adding it to the PREFIXED_HISTOGRAM_INTERNAL.

I suggest either removing this origin or adding it to the histogram to protect
against such a regression, and maybe get rid of
RecordLocalPredictorTimeUntilUsed and use RecordTimeUntilUsed instead (Though
suppose the histogram is a bit different from the others, so may be best to just
keep them separate).

[gavinp, 2012/10/10 18:14:56]

Very good catch. This kind of subtlety is what's made writing this CL and
revising it hard. :/ And I'm sad, but not surprised to hear I missed a neat
special case.

I don't like renaming these to not predictor names, since then someone might
think they should be called for different origins.

Besides adding this origin text, the other thing the macros in
prerender_histograms.cc do is to mix in the experiment, which of course doesn't
make sense for the predictor either (and when present, it's only confusing).

So I'm removing this origin, and moving these histograms out of
prerender_histograms.cc.

+  ORIGIN_NONE = 7,
+  ORIGIN_MAX,
 };
 
 // Return a human-readable name for |origin|.