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

chrome/browser/predictors/resource_prefetch_predictor_browsertest.cc

Index: chrome/browser/predictors/resource_prefetch_predictor_browsertest.cc
diff --git a/chrome/browser/predictors/resource_prefetch_predictor_browsertest.cc b/chrome/browser/predictors/resource_prefetch_predictor_browsertest.cc
index cf305a34d5db42f9f6a0ccc378f0eac0ef57cfe6..4b6a681486df0d4ab3f1f84052205bd135eed63a 100644
--- a/chrome/browser/predictors/resource_prefetch_predictor_browsertest.cc
+++ b/chrome/browser/predictors/resource_prefetch_predictor_browsertest.cc
@@ -8,6 +8,7 @@
 
 #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"
@@ -289,9 +290,13 @@ class ResourcePrefetchPredictorBrowserTest : public InProcessBrowserTest {
   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 {