Delete obsolete prerender experiment code

chrome/browser/prerender/prerender_manager.h

Index: chrome/browser/prerender/prerender_manager.h
diff --git a/chrome/browser/prerender/prerender_manager.h b/chrome/browser/prerender/prerender_manager.h
index 1a7f53d005864e7af24c84880ab745ef68fe329f..e359dde3c87fe64ed5e7aa8d6c95fcf81b8c4840 100644
--- a/chrome/browser/prerender/prerender_manager.h
+++ b/chrome/browser/prerender/prerender_manager.h
@@ -80,19 +80,11 @@ class PrerenderManager : public content::NotificationObserver,
                          public KeyedService,
                          public MediaCaptureDevicesDispatcher::Observer {
  public:
-  // NOTE: New values need to be appended, since they are used in histograms.

[droger, 2016/11/25 13:12:42]

The histogram no longer exist in the code, but it still exists in histogram.xml,
marked as deprecated since 2012.
I did not delete the histogram from histogram.xml, so that the data remains
accessible from UMA queries, but we could consider deleting it maybe.

[mattcary, 2016/11/25 13:21:07]

Does this change does mean that is will be nearly impossible to interpret the
old histograms, as the enum reference is gone, or does  uma save the enum value
names?

[droger, 2016/11/25 13:44:36]

Looking at histogram in UMA does not use the enum from C++, it has it's own enum
in histograms.xml (which is basically a copy of this one).

The C++ enum was only used to upload data from the client, but in this case, it
is not even used at all currently since the histogram is already deleted from
the C++ code.

[pasko, 2016/11/25 14:19:31]

yeah, UMA dashbnoard does not know about enums in C++, it renders from the
uploaded ints/buckets and histograms.xml.

   enum PrerenderManagerMode {
-    PRERENDER_MODE_DISABLED = 0,
-    PRERENDER_MODE_ENABLED = 1,
-    PRERENDER_MODE_EXPERIMENT_CONTROL_GROUP = 2,
-    PRERENDER_MODE_EXPERIMENT_PRERENDER_GROUP = 3,
-    // Obsolete: PRERENDER_MODE_EXPERIMENT_5MIN_TTL_GROUP = 4,
-    PRERENDER_MODE_EXPERIMENT_NO_USE_GROUP = 5,
-    PRERENDER_MODE_EXPERIMENT_MULTI_PRERENDER_GROUP = 6,
-    PRERENDER_MODE_EXPERIMENT_15MIN_TTL_GROUP = 7,
-    // Obsolete: PRERENDER_MODE_EXPERIMENT_MATCH_COMPLETE_GROUP = 8,
-    PRERENDER_MODE_NOSTATE_PREFETCH = 9,
-    PRERENDER_MODE_MAX = 10
+    PRERENDER_MODE_DISABLED,
+    PRERENDER_MODE_ENABLED,
+    PRERENDER_MODE_EXPERIMENT_NO_USE_GROUP,
+    PRERENDER_MODE_NOSTATE_PREFETCH
   };
 
   // One or more of these flags must be passed to ClearData() to specify just
@@ -230,8 +222,6 @@ class PrerenderManager : public content::NotificationObserver,
   static PrerenderManagerMode GetMode();
   static void SetMode(PrerenderManagerMode mode);
   static bool IsPrerenderingPossible();
-  static bool ActuallyPrerendering();
-  static bool IsControlGroup();
   static bool IsNoUseGroup();
   static bool IsNoStatePrefetch();