Added the list of active field groups to NetLog dump

This CL adds a list of active field groups to NetLog dump. The string
representation is human readable, in the form of trial_name:group_name.

BUG=426474

Review URL: https://codereview.chromium.org/671253002

Cr-Commit-Position: refs/heads/master@{#301104}

chrome/browser/resources/net_internals/import_view.html

@@ -63,6 +63,12 @@
       <th>Command line</th>
       <td><pre jscontent="constants.clientInfo.command_line"></pre></td>
     </tr>
+    <tr>
+      <th>Active Field Trial Groups</th>
+      <td>
+        <span jsselect="constants.activeFieldTrialGroups" jscontent="$this + ' '"></span>
+      </td>
+    </tr>
     <tr>
       <th>User comments</th>
       <td>

net/base/net_log_logger.cc

@@ -9,6 +9,7 @@
 #include "base/json/json_writer.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/metrics/field_trial.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/values.h"
 #include "net/base/address_family.h"
@@ -258,6 +259,20 @@ base::DictionaryValue* NetLogLogger::GetConstants() {
   // Provide a default empty value for compatibility.
   constants_dict->Set("clientInfo", new base::DictionaryValue());
 
+  // Add a list of active field experiments.
+  {
+    base::FieldTrial::ActiveGroups active_groups;
+    base::FieldTrialList::GetActiveFieldTrialGroups(&active_groups);
+    base::ListValue* field_trial_groups = new base::ListValue();
+    for (base::FieldTrial::ActiveGroups::const_iterator it =
+             active_groups.begin();
+         it != active_groups.end(); ++it) {
+      field_trial_groups->AppendString(it->trial_name + ":" +
+                                       it->group_name);
+    }
+    constants_dict->Set("activeFieldTrialGroups", field_trial_groups);
+  }
+
   return constants_dict;
 }