Add the PlatformFieldTrials Class

This is part 4 of 6 for FieldTrial refactoring for WebView

Part 1: https://chromium-review.googlesource.com/c/562098/
Part 2: https://chromium-review.googlesource.com/c/561920/
Part 3: https://chromium-review.googlesource.com/c/561922/
Part 4: https://chromium-review.googlesource.com/c/561980/
Part 5: https://chromium-review.googlesource.com/c/562417/
Part 6: https://chromium-review.googlesource.com/c/562021/

This CL adds a new class, PlatformFieldTrials, that the 
Chrome specific ChromeBrowserFieldTrials is now
derived from. Its implementation is currently just stubs
as they shouldn't be necessary for WebView.

BUG=678288

Change-Id: I5d7b1b77f5af39594977a5bd7ed66bf51cb63628
Reviewed-on: https://chromium-review.googlesource.com/561980
Commit-Queue: Kyle Milka <kmilka@google.com>
Reviewed-by: Alexei Svitkine (slow) <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488383}

chrome/browser/chrome_browser_field_trials.h

@@ -6,26 +6,22 @@
 #define CHROME_BROWSER_CHROME_BROWSER_FIELD_TRIALS_H_
 
 #include "base/macros.h"
+#include "components/variations/platform_field_trials.h"
 
 namespace base {
 class FeatureList;
 }
 
-class ChromeBrowserFieldTrials {
+class ChromeBrowserFieldTrials : public variations::PlatformFieldTrials {
  public:
   ChromeBrowserFieldTrials();
-  ~ChromeBrowserFieldTrials();
-
-  void SetupFieldTrials();
-
-  // Create field trials that will control feature list features. This should be
-  // called during the same timing window as
-  // FeatureList::AssociateReportingFieldTrial. |has_seed| indicates that the
-  // variations service used a seed to create field trials. This can be used to
-  // prevent associating a field trial with a feature that you expect to be
-  // controlled by the variations seed.
-  void SetupFeatureControllingFieldTrials(bool has_seed,
-                                          base::FeatureList* feature_list);
+  ~ChromeBrowserFieldTrials() override;
+
+  // variations::PlatformFieldTrials:
+  void SetupFieldTrials() override;
+  void SetupFeatureControllingFieldTrials(
+      bool has_seed,
+      base::FeatureList* feature_list) override;
 
  private:
   // Instantiates dynamic trials by querying their state, to ensure they get

components/variations/BUILD.gn

@@ -30,6 +30,7 @@ static_library("variations") {
     "metrics.h",
     "metrics_util.cc",
     "metrics_util.h",
+    "platform_field_trials.h",
     "pref_names.cc",
     "pref_names.h",
     "processed_study.cc",

components/variations/platform_field_trials.h

+// Copyright 2017 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 COMPONENTS_VARIATIONS_PLATFORM_FIELD_TRIALS_H_
+#define COMPONENTS_VARIATIONS_PLATFORM_FIELD_TRIALS_H_
+
+#include "base/metrics/field_trial.h"
+
+namespace variations {
+
+// Infrastructure for setting up platform specific field trials. Chrome and
+// WebView make use through their corresponding subclasses.
+class PlatformFieldTrials {
+ public:
+  PlatformFieldTrials(){};
+  virtual ~PlatformFieldTrials(){};
+
+  // Set up field trials for a specific platform.
+  virtual void SetupFieldTrials() = 0;
+
+  // Create field trials that will control feature list features. This should be
+  // called during the same timing window as
+  // FeatureList::AssociateReportingFieldTrial. |has_seed| indicates that the
+  // variations service used a seed to create field trials. This can be used to
+  // prevent associating a field trial with a feature that you expect to be
+  // controlled by the variations seed.
+  virtual void SetupFeatureControllingFieldTrials(
+      bool has_seed,
+      base::FeatureList* feature_list) = 0;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(PlatformFieldTrials);
+};
+
+}  // namespace variations
+
+#endif  // COMPONENTS_VARIATIONS_PLATFORM_FIELD_TRIALS_H_