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

chrome/browser/extensions/activity_log/activity_log_browsertest.cc

Index: chrome/browser/extensions/activity_log/activity_log_browsertest.cc
diff --git a/chrome/browser/extensions/activity_log/activity_log_browsertest.cc b/chrome/browser/extensions/activity_log/activity_log_browsertest.cc
index e9248b0041ba9cea9bb867bb3c4ee2b11871a215..b35b4a06deef0aa55def7a11b3f2422bacc138cf 100644
--- a/chrome/browser/extensions/activity_log/activity_log_browsertest.cc
+++ b/chrome/browser/extensions/activity_log/activity_log_browsertest.cc
@@ -36,8 +36,12 @@ class ActivityLogPrerenderTest : public ExtensionApiTest {
   void SetUpCommandLine(base::CommandLine* command_line) override {
     ExtensionBrowserTest::SetUpCommandLine(command_line);
     command_line->AppendSwitch(switches::kEnableExtensionActivityLogging);
-    command_line->AppendSwitchASCII(switches::kPrerenderMode,
-                                    switches::kPrerenderModeSwitchValueEnabled);
+  }
+
+  void SetUpOnMainThread() override {
+    ExtensionApiTest::SetUpOnMainThread();
+    prerender::PrerenderManager::SetMode(
+        prerender::PrerenderManager::PRERENDER_MODE_ENABLED);
   }
 
   static void Prerender_Arguments(