Delete obsolete prerender experiment code

chrome/browser/prerender/prerender_manager.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_MANAGER_H_
 #define CHROME_BROWSER_PRERENDER_PRERENDER_MANAGER_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 62 matching lines @@
 class PrerenderLocalPredictor;
 
 // PrerenderManager is responsible for initiating and keeping prerendered
 // views of web pages. All methods must be called on the UI thread unless
 // indicated otherwise.
 class PrerenderManager : public content::NotificationObserver,
                          public content::RenderProcessHostObserver,
                          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
   // what data to clear.  See function declaration for more information.
   enum ClearFlags {
     CLEAR_PRERENDER_CONTENTS = 0x1 << 0,
     CLEAR_PRERENDER_HISTORY = 0x1 << 1,
     CLEAR_MAX = 0x1 << 2
   };
 
@@ skipping 117 matching lines @@
   // Must not be called for prefetch loads (which are never rendered anyway).
   // |is_no_store| must be true if the main resource has a "no-store" cache
   // control HTTP header.
   void RecordFirstContentfulPaint(const GURL& url,
                                   bool is_no_store,
                                   base::TimeDelta time);
 
   static PrerenderManagerMode GetMode();
   static void SetMode(PrerenderManagerMode mode);
   static bool IsPrerenderingPossible();
-  static bool ActuallyPrerendering();
-  static bool IsControlGroup();
   static bool IsNoUseGroup();
   static bool IsNoStatePrefetch();
 
   // Query the list of current prerender pages to see if the given web contents
   // is prerendering a page. The optional parameter |origin| is an output
   // parameter which, if a prerender is found, is set to the Origin of the
   // prerender |web_contents|.
   bool IsWebContentsPrerendering(const content::WebContents* web_contents,
                                  Origin* origin) const;
 
@@ skipping 368 matching lines @@
   std::unique_ptr<base::TickClock> tick_clock_;
 
   base::WeakPtrFactory<PrerenderManager> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PrerenderManager);
 };
 
 }  // namespace prerender
 
 #endif  // CHROME_BROWSER_PRERENDER_PRERENDER_MANAGER_H_