Fix prerender histograms for multiple prerender case.

chrome/browser/prerender/prerender_origin.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_PRERENDER_PRERENDER_ORIGIN_H_
 #define CHROME_BROWSER_PRERENDER_PRERENDER_ORIGIN_H_
 
 namespace prerender {
 
 // Origin indicates what caused the prerender.
 // NOTE: New values need to be appended, since they are used in histograms.
 enum Origin {
   ORIGIN_LINK_REL_PRERENDER = 0,
   // Obsolete: ORIGIN_OMNIBOX_ORIGINAL = 1,
   ORIGIN_GWS_PRERENDER = 2,
   // 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|.
 const char* NameFromOrigin(Origin origin);
 
 }  // namespace prerender
 
 #endif  // CHROME_BROWSER_PRERENDER_PRERENDER_ORIGIN_H_