[Prerender] Get the prerender mode from Finch field trial.

chrome/browser/prerender/prerender_field_trial.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_FIELD_TRIAL_H_
 #define CHROME_BROWSER_PRERENDER_PRERENDER_FIELD_TRIAL_H_
 
+#include "base/feature_list.h"
+
 class Profile;
 
-namespace base {
-class CommandLine;
-}
-
 namespace prerender {
 
-// Parse the --prerender= command line switch, which controls prerendering. If
-// the switch is unset or is set to "auto" then the user is assigned to a
-// field trial.
-void ConfigurePrerender(const base::CommandLine& command_line);
+extern const base::Feature kNoStatePrefetchFeature;

[pasko, 2016/12/01 16:47:58]

do we want the feature in about:flags as well? (sounds handy to me)

otherwise, this extern const is used only for testing, so it would be preferable
to explicitly say so:

extern const& base::Feature GetNoStatePrefetchFeatureForTesting()

[droger, 2016/12/01 17:17:40]

It seems that there is one (and only one) feature that uses about_flags in
combination with finch parameters (i.e. FEATURE_WITH_VARIATIONS_VALUE_TYPE)
which is ntp_snippets.

I can look into it and see if that works for us.

[droger, 2016/12/02 16:51:23]

I looked into this, and I have a working prototype. That would better be done in
another CL though.

Note that adding the feature to about://flags does not allow to set its value
from the command line either :(

+
+// Configures prerender using kNoStatePrefetchFeature and field trial
+// parameters.
+void ConfigurePrerender();
 
 // Returns true if the user has opted in or has been opted in to the
 // prerendering from Omnibox experiment.
 bool IsOmniboxEnabled(Profile* profile);
 
 }  // namespace prerender
 
 #endif  // CHROME_BROWSER_PRERENDER_PRERENDER_FIELD_TRIAL_H_