predictors: Move the configuration of speculative prefetch to field trials.

chrome/browser/predictors/resource_prefetch_predictor_browsertest.cc

 // Copyright 2016 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.
 
 #include <stddef.h>
 
 #include <set>
 
 #include "base/command_line.h"
 #include "base/memory/ptr_util.h"
+#include "base/strings/string_util.h"
 #include "chrome/browser/browsing_data/browsing_data_helper.h"
 #include "chrome/browser/browsing_data/browsing_data_remover.h"
 #include "chrome/browser/browsing_data/browsing_data_remover_factory.h"
 #include "chrome/browser/predictors/resource_prefetch_predictor.h"
 #include "chrome/browser/predictors/resource_prefetch_predictor_factory.h"
 #include "chrome/browser/predictors/resource_prefetch_predictor_test_util.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
 #include "chrome/common/chrome_switches.h"
@@ skipping 261 matching lines @@
   GURL main_frame_url_;
 
   DISALLOW_COPY_AND_ASSIGN(PrefetchingObserver);
 };
 
 class ResourcePrefetchPredictorBrowserTest : public InProcessBrowserTest {
  protected:
   using URLRequestSummary = ResourcePrefetchPredictor::URLRequestSummary;
 
   void SetUpCommandLine(base::CommandLine* command_line) override {
-    command_line->AppendSwitchASCII(
-        switches::kSpeculativeResourcePrefetching,
-        switches::kSpeculativeResourcePrefetchingEnabledExternal);
+    command_line->AppendSwitchASCII("force-fieldtrials", "trial/group");
+    std::string parameter = base::StringPrintf(
+        "trial.group:%s/%s", kModeParamName, kExternalPrefetchingMode);
+    command_line->AppendSwitchASCII("force-fieldtrial-params", parameter);
+    std::string enabled_feature = base::StringPrintf(
+        "%s<trial", kSpeculativeResourcePrefetchingFeatureName);
+    command_line->AppendSwitchASCII("enable-features", enabled_feature);

[droger, 2017/01/25 17:27:11]

Optional:

You may be able to do this without using the command line.

See
https://codereview.chromium.org/2537503002/diff/180001/chrome/browser/prerend...
for an example.

Not sure if it is better though.

[Benoit L, 2017/01/26 09:58:15]

This works in unit tests, but not in browser tests, as the dependency is not
compiled for browser tests.
And this is actually slightly better, as we end up testing the two ways of
enabling the predictor, with the unit tests and browser tests.

   }
 
   void SetUpOnMainThread() override {
     // Resolving all hosts to local allows us to have
     // cross domains navigations (matching url_visit_count_, etc).
     host_resolver()->AddRule("*", "127.0.0.1");
     embedded_test_server()->RegisterRequestHandler(
         base::Bind(&ResourcePrefetchPredictorBrowserTest::HandleRedirectRequest,
                    base::Unretained(this)));
     embedded_test_server()->RegisterRequestHandler(
@@ skipping 562 matching lines @@
   TryToPrefetchURL(initial_url);
   NavigateToURLAndCheckSubresources(initial_url);
   ClearCache();
   // But the predictor database contains all subresources for the endpoint url
   // so this prefetch works.
   PrefetchURL(GetURL(kHtmlSubresourcesPath));
   NavigateToURLAndCheckSubresourcesAllCached(GetURL(kHtmlSubresourcesPath));
 }
 
 }  // namespace predictors